GB2624249A

GB2624249A - A trailer braking system

Info

Publication number: GB2624249A
Application number: GB2216957.7A
Authority: GB
Inventors: Crawley James
Original assignee: Haldex Brake Products AB
Current assignee: Haldex Brake Products AB
Priority date: 2022-11-14
Filing date: 2022-11-14
Publication date: 2024-05-15
Also published as: GB202216957D0

Abstract

A trailer braking system and a method, comprising: an electronic control unit (ECU 50, fig. 4), a spring brake control assembly (30; fig. 4) for controlling spring brake(s) (20; fig. 4), a supply line 12, a park valve 60, and a park override valve 70. The park valve 60 has a first position in which the supply line connects to the control assembly and/or spring brake(s), and a second position in which air is prevented from reaching the control assembly or brakes. The park override valve 70 is connected to a park valve control port 60e and the park override valve is biased to a first position in which the control port is connected to an exhaust, and is electrically operable to a second position in which the control port is connected to a supply of air (e.g. the supply line 12, or trailer reservoir 40), e.g. in response to an electronic signal from the ECU, which itself is in response to a signal from an external source. There may also be a mobiliser valve (80; fig. 4), an emergency override valve (90; fig. 4) and a park valve sensor (66; fig. 4) indicating the position of the park valve.

Description

A TRAILER BRAKING SYSTEM

FIELD

Embodiments of the present invention relate to a trailer braking system. The braking system is primarily for use in a trailer that is configured to be connected and towed by a towing vehicle.

BACKGROUND

Heavy goods vehicles are well known. Typically, a trailer is connected to a towing vehicle and goods are transported from a starting position to a delivery point. Sometimes a route may include transport on a ferry or other vehicle transporter. In such a situation, the driver of the vehicle will leave the vehicle and a ferry operator may be responsible for moving the vehicle to a desired location (e.g. on the ferry). A shunt valve may be provided which allows such an operator to unlock a parking brake with a manual button push and supply of air. However, such operators often do not complete this step and, as such, the trailer may remain with its parking brakes applied while the ferry operator moves it around, which can cause damage and wear on the tyres (as the vehicle is dragged rather than the wheels being permitted to rotate). Alternatively, the driver may leave the parking brake in a released position if they anticipate the ferry operator may not use the shunt valve meaning the vehicle is abandoned in a potentially unsafe condition.

Embodiments of the present invention seek to alleviate one or more issues associated with the prior art.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the invention, we provide a trailer braking system including: an electronic control unit which is operable to send and receive signals, a spring brake control assembly which controls a spring brake actuator, the spring brake actuator having an applied state in which a brake force is applied to a wheel and a released state in which no brake force is applied to the wheel, a supply line for supplying air to the spring brake control assembly and! or the spring brake actuator, a park valve which has a first position in which the supply line is open and connects to the spring brake control assembly and / or the spring brake actuator and the spring brake actuator is controlled by the spring brake assembly, and a second position in which the supply line is closed and air is prevented from reaching the spring brake control assembly or the spring brake actuator, and a park override valve which is connected to a control port of the park valve and the park override valve is biased to a first position in which the control port is connected to an exhaust and has a second position in which the control port is connected to a supply of air, wherein the park override valve is electrically operable to its second position in response to an electronic signal.

The trailer braking system may include a trailer reservoir for storing air.

The park override valve may include a connection to the supply line. When the park override valve is in its second position, the supply line may be connected to the control port of the park valve.

The park override valve may include a first port which is connected to the control port of the park valve, a second port which may be connected to the supply line and a third port which may be the exhaust! vents to atmosphere.

The park override valve may include a connection to the trailer reservoir. When the park override valve is in its second position, the trailer reservoir may be connected to the control port of the park valve.

The park override valve may include a first port which may be connected to the control port of the park valve, a second port which may be connected to the trailer reservoir and a third port which may be the exhaust! vents to atmosphere.

The trailer braking system may include a mobiliser valve. The mobiliser valve may have a first position in which air in the supply line may be permitted to the spring brake control assembly and a second position in which air in the supply line may be prevented from reaching the spring brake assembly. The mobiliser valve may be pneumatically driven via a control port.

The trailer braking system may include an emergency override valve which may be connected to the control port of the mobiliser valve and may be controlled by the electronic control unit. The emergency override valve may have a first position in which the supply line may be connected to the spring brake control assembly and the control port of the mobiliser valve and a second position in which the trailer reservoir may be connected to the spring brake control assembly and the control port of the mobiliser valve.

The mobiliser valve may be controllable, between its first and second positions, electrically via signals from the electronic control unit.

The electronic signal that controls operation of the park override valve may be generated by the electronic control unit, and optionally the electronic signal may be generated on receipt of a signal from an electronic control unit on a towing vehicle.

The park valve may include a release piston at its control port which may be connected to the park override valve, so that the park valve is moved to its first position, when the park override valve is in its second position, by the release piston. The release piston may be a single direction push piston.

The trailer braking system may include a shunt valve which may have a first condition in which the supply line may be connected to the park valve and a second condition in which the trailer reservoir may be connected to the park valve.

A park valve sensor may be provided which may output a signal indicative of the position of the park valve. The park valve sensor may be a reed switch which has two states. It may output a constant signal when the park valve is in one of its positions and no signal when the park valve is the other one of its positions.

According to a second aspect of the invention, we provide a method of operating a trailer braking system according to any one of the preceding claims, including: the electronic control unit receiving a signal from an external source to release the park valve, and generating a signal for sending to the park override valve, actuating the park override valve to its second position and connecting a supply of air to the control port of the park valve, causing the park valve to move to its first position, controlling the spring brake control assembly to enable movement of the spring brake actuator to its released state.

After actuating the park override valve, the method may include the electronic control unit sending a mobilise signal and actuating the mobiliser valve to its first position. The mobilise signal may be sent to the emergency override valve, actuating the emergency override valve to its second position. The mobilise signal may be sent to the mobiliser valve and actuating it to its first position.

BRIEF DESCRIPTION OF THE FIGURES

In order that the present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a view of a trailer braking system, embodying the present disclosure; FIGURE 2 Is a view of an alternative trailer braking system, embodying the present disclosure; FIGURE 3 is another view of a trailer braking system, embodying the present disclosure; and FIGURE 4 is a general schematic of an example trailer.

DETAILED DESCRIPTION OF THE DISCLOSURE

With reference to the figures, a trailer braking system 10 is illustrated. The trailer braking system 10 has a supply line 12 which is connected / connectable to a towing vehicle (not shown). The towing vehicle will provide a source of continuous fluid (usually air) to the trailer when connected (i.e. the towing vehicle may have a generator that feeds the supply line 12 while the trailer is connected). The trailer braking system 10 also includes a spring brake control assembly 30, a spring brake actuator 20, an electronic control unit (ECU) 50, a park valve 60 and a park override valve 70.

In some embodiments, the trailer braking system 10 has a second fluid line which is connected / connectable to the towing vehicle. The second line is a control line 14, which is pressurised when a braking demand is generated in the towing vehicle (for example, when a driver puts their foot on a brake pedal). The control line 14 is only pressurised in the event of a brake demand in order to signal to the trailer braking system 10 that service braking is required.

The spring brake actuator 20 is operable to provide a braking force to an associated wheel 22 (the trailer may have multiple spring brake actuators 20 -six are shown in figure 4 -each of which provides a braking force to a wheel 22). The spring brake actuator 20 has an applied state and a released state. In the applied state, a brake force is applied to a wheel 22. In the released state, no (or negligible) brake force is applied to the wheel 22.

The spring brake actuator 20 is configured to move to its released state when the air pressure supplied to the actuator 20 is above a predetermined threshold. When the pressure is below the threshold, the spring brake actuator 20 applies the braking force to the wheel 22 automatically. In other words, when the pressure supplied to the spring brake actuator 20 is above the threshold, an internal spring is held off and no brake force is generated. However, below that threshold the spring is unpressured and thus, the spring generates the required braking force on the wheel 22.

The operation of the spring brake actuator 20 is controlled by the spring brake control assembly 30.

In other words, the spring brake control assembly 30 is operable to control the pressure/air permitted to the spring brake actuator 20. Thus, it controls the application and release of the spring brake actuator 20. The supply line 12 may connect to the spring brake control assembly 30, which directly controls the air travelling to the spring brake control assembly 20. Alternatively, a relay valve may be included and the spring brake control assembly 30 provides pressure to a control and the supply line 12 pressure permitted through the relay valve to the spring brake actuator 20 is proportional to the control pressure. Thus, the supply line 12 supplies air to the spring brake actuator 20 and / or to the spring brake control assembly 30.

The electronic control unit (ECU) 50 is operable to send and receive signals (for example, electrical signals). In the present example, the ECU 50 is connected to the towing vehicle alongside the pneumatic connections provided by the supply and control lines 12, 14. Such a connection may provide power to the ECU 50 and electronic signals relating to the braking situation of the vehicle and whether braking input is required from the trailer braking system 10 (such signals may be sent via a CAN bus connection or other standard that may be applicable). Further, the ECU 50 may be able to control the service brakes directly when a braking demand from the towing vehicle is received. Alongside, the ECU 50 may be capable of operating and applying the service brakes of the trailer autonomously (for example, if the ECU 50 identifies that the trailer is unstable or in danger of rolling, it may be able to apply the service brakes without input from the towing vehicle to prevent / reduce the instability).

As mentioned above, the trailer braking system 10 also includes the park valve 60 and the park override valve 70. The park valve 60 has a first position and a second position. In its first position, the supply line 12 is open and connects to the spring brake control assembly 30 and! or the spring brake actuator 20. Thus, in this position the spring brake actuator 20 is controlled by the spring brake control assembly 30 (as normal). In its second position, the supply line 12 is closed and air is prevented from reaching the spring brake control assembly 30 and / or the spring brake actuator 20. In other words, the park valve 60 functions to selectively isolate the supply line 12 from the spring brake control assembly 30 and / or the spring brake actuator 20 (so that the spring brake actuator 20 is automatically applied).

The park valve 60 provides a manually actuated valve which once activated, isolates the supply line 12 from the remainder of the braking system. This provides redundancy in ensuring the spring brakes remain applied when the trailer is parked (i.e. stopped for an extended period of time and not travelling on the road) and the park valve 60 is applied. As discussed above, when the pressure at the spring brake actuator 20 drops below a certain level (for example, when the supply line 12 is disconnected from the towing vehicle / a source of air), the brakes will be activated automatically.

In embodiments, the park valve 60 has three ports. A first port 60a is connected to the supply line 12 (coming from the connection to the towing vehicle). A second port 60b is connected to the spring brake control assembly 30 (and, potentially, directly to the spring brake actuator 20 also). A third port 60c is connected to a vent. Thus, in its first position (i.e. when the trailer is in a normal operating state or travelling, for example) the first port 60a is connected to the second port 60b and the third port 60c is closed. In its second position (i.e. when the trailer is parked) the first port 60a is blocked and the second port 60b is connected to the third port 60c (and, thus, venting to atmosphere). A manual button / actuator 60d is provided -this maybe external to the trailer, so that an operator can access the valve and apply the park valve 60 as necessary. Further, the park valve 60 includes a control port 60e -this is discussed in more detail below.

The park override valve 70 functions to provide an override of the park valve 60-essentially allowing the ECU 50 to cause the park valve 60 to return to its first position. In this example, the park override valve 70 is an electronically operated solenoid with two positions! states.

The park override valve 70 is connected to the control port 60e of the park valve 60. In its first position, the park override valve 70 connects the control port 60e to an exhaust port 70c. In its second position, the park override valve 70 connects the control port 60e to a supply of air. Thus, in the second position, the park override valve 70 causes the park valve 60 to move to its first position.

The park override valve 70 is biased to its first position (i.e. the control port 60e is exhausted and has no effect on the position of the park valve 60). A biasing element 71 (such as a spring) may be present to bias the park override valve 70 to its first position.

Following a signal from the ECU 50, the park override valve 70 moves to connect air to the park valve 60. The supply of air may be from a direct connection between the park override valve 70 and the supply line 12 (see figure 1, for example). Thus, this option relies on the supply line 12 being connected to a source of air when the park override valve 70 is used / activated.

The braking system 10 may include a trailer reservoir 40 for storing air. During normal use (when a towing vehicle is connected to the trailer and providing air to the supply line), the trailer reservoir 40 is kept full. This ensures that the trailer has an auxiliary supply of air that can be used if required (for example, in an emergency situation where towing vehicle air is not available but it is safer to hold the spring brake actuator 20 in a released state, so an alternative source of air is required). In such a system 10, the supply of air to the park override valve 70 may come from a connection to the trailer reservoir 40 (illustrated in figure 2). Thus, this option does not require an external source of air when the park override valve 70 is used / activated.

In some embodiments, the park override valve 70 has three ports and an electrically operable control 70d. The control port 60e of the park valve 60 is connected to a first port 70a. The supply of air (i.e. the supply line 12 or the trailer reservoir 40) is connected to a second port 70b. The exhaust port 70c provides a third port, which provides an outlet to atmosphere.

A first arrangement of the park override valve 70 is illustrated in figure 1. In this example, the second port 70b is connected to the supply line 12. The connection is upstream of any of the valves / components forming the remainder of the braking system 10. Thus, if a towing vehicle is connected to the trailer and the park valve 60 must be released, the air on the supply line 12 from the towing vehicle is used.

The example illustrated in figure 3 includes a park override valve 70 with the same ports / connections as discussed above in relation to figure 1. The control port 60e of the park valve 60 includes a release piston 61. The release piston 61 may be a single action piston, which is operable to push the park valve 60 to its first position when connected to air from the park override valve 70. It should be appreciated that the release piston 61 may also be used with the second arrangement outlined below also.

A second arrangement of the park override valve 70 is illustrated in figure 2. The difference between the example system illustrated in figure 1 and the system illustrated in figure 2 is the connection of the park override valve 70 to a supply of air. As such, the reference numbers have been maintained across both examples to aid understanding. In this second example, the second port 70b is connected to the trailer reservoir 40 (the first port 70a is still connected to the control port 60e of the park valve 60 and the third port 70c provides the exhaust). Thus, when the park override valve 70 moves to its second position, the trailer reservoir 40 (and the air within the reservoir) is used to control the park valve 60 and move it to its first position.

In some embodiments, the trailer braking system 10 includes a mobiliser valve 80. The mobiliser valve 80 has a first and second position. In its first position, air in the supply line 12 is permitted to the spring brake control assembly 30. In other words, when the mobiliser valve 80 is in its first position, the spring brake control assembly 30 is operable as normal to allow or prevent air reaching the spring brake actuator 20 as controlled. In its second position, air in the supply line 12 is prevented from reaching the spring brake control assembly 30. In other words, the spring brake control assembly 30 is not operable at all because the air cannot reach the control assembly 30 and cannot be permitted to the spring brake actuator 20 either. Essentially, the mobiliser valve 80 prevents unauthorised or unauthenficated removal of the trailer. Even if the park valve 60 is manually moved to its first position (and air is permitted along the supply line 12), if the ECU 50 has not released the mobiliser valve 80 then the spring brake actuators 20 cannot be released (and the trailer cannot be moved without risking damage to the wheels / tyres).

The mobiliser valve 80 may be controllable, between its first and second positions, electrically via signals from the electronic control unit 50. In some embodiments, the mobiliser valve 80 is pneumatically driven via a control port. In such an embodiment, an emergency override valve 90 may be provided. The emergency override valve 90 is connected to the control port of the mobiliser valve 80 and is controlled by the electronic control unit 50. In some embodiments, the emergency override valve 90 has a first and second position. In the first position, the supply line 12 is connected to the spring brake control assembly 30 and the control port of the mobiliser valve 80. In the second position, the trailer reservoir 40 is connected to the spring brake control assembly 30 and the control port of the mobiliser valve 80.

In some embodiments, the trailer braking system 10 includes a shunt valve 64 which has a first and second condition. In its first condition, the supply line 12 is connected to the park valve 60, and in its second condition, the trailer reservoir 40 is connected to the park valve 60.

In some embodiments, a park valve sensor 66 is provided which outputs a signal indicative of the position of the park valve 60. The park valve sensor 66 may be a reed switch which has two states. The reed switch then outputs a constant signal when the park valve 60 is in one of its positions and no signal when the park valve 60 is the other one of its positions. Thus, feedback from the sensor 66 is provided to the ECU 50, so that the actual position of the park valve 60 is known.

The operation of the system 10 will now be discussed in more detail. When the trailer is connected to the towing vehicle (i.e. the supply line 12 is connected to a constant source of air and the control line 14 is connected to the brake demand output), the system 10 is in a "normal" and travelling state. In such a state, the park valve 60 is in its first position, so that air on the supply reaches the spring brake control assembly 30. The spring brake control assembly 30 is configured to allow air to the spring brake actuator(s) 20 and the spring brakes are held off automatically. Thus, the trailer is in a state in which it can be towed normally.

If and when the trailer is stopped (e.g. if the vehicle has reached its destination), the towing vehicle may be disconnected from the supply line 12 and the control line 14 (as part of an unhitching process). The lack of air from the towing vehicle on the supply line 12 will cause the spring brake control assembly 30 to move to its second position automatically. Thus, the spring brake actuator(s) 20 will move to their applied position.

When the trailer is stopped, the park valve 60 may then be actuated manually. For example, the driver may leave the cab and pull the park valve 60 to its second position. In this position, the park valve 60 prevents air from reaching the spring brake control assembly 30 and the spring brake actuator 20. Further, the supply line between the park valve 60 and the spring brake control assembly 30 / spring brake actuator 20 (referenced as portion 12a) is exhausted (through the third / vent port 60c) When the trailer requires moving, the shunt valve 70 may be used. If the trailer is being moved without a towing vehicle (i.e. a supply of air) then the shunt valve 70 must be used to unlock the spring brakes because it connects the trailer reservoir 40 to the spring brakes (and allows them to release). However, there are situations in which the shunt valve 70 is not correctly actuated prior to moving the trailer. When the trailer is on a ferry or at a port, for example, the operators often will not actuate the shunt valve 70 before moving a trailer. Further, in the event that a shunting towing vehicle is an autonomous vehicle, there may not be an operator available to actuate the shunt valve 70. The park override valve 70 provides an alternative.

In embodiments, the trailer is connected to a towing vehicle (which may be an autonomous vehicle or human operated). The method of using the park override valve 60 includes the following steps: the electronic control unit 50 receives a signal from an external source to release the park valve 60, and generates a signal for the park override valve 70. The park override valve 70 is actuated to its second position and connects a supply of air to the control port 60e of the park valve 60. This causes the park valve 60 to move to its first position (i.e. connecting the supply line 12 to the spring brake control assembly 30). Thus, it controls the spring brake control assembly 30 to enable movement of the spring brake actuator 20 to its released state.

In systems that include a mobiliser valve 80, the method further includes the electronic control unit 50 sending a mobilise signal and actuating the mobiliser valve 80 to its first position. This signal is sent after actuating the park override valve 70. Alternatively, if the system includes the emergency override valve 90, then the mobilise signal is sent to the emergency override valve 90, which in turn connects the mobiliser valve 80 to the trailer reservoir and, thus, actuating the mobiliser valve 80 to its second position.

It should be appreciated that the electronic signal that controls operation of the park override valve 70 is generated by the electronic control unit 50. In some embodiments, the electronic signal is generated (by the ECU 50) on receipt of a signal from an electronic control unit on a towing vehicle.

Thus, an option is provided for releasing the park valve 60 through a signal generated at the ECU 50 and without a manual intervention to the trailer. The driver in the cab could initiate a signal from a button or an autonomous towing vehicle could be programmed to request that the ECU 50 released the park valve 60 when it is connected successfully.

In some embodiments, the park valve 60 includes a supply line return piston 62 at a second control port which is connected to the supply line 12 downstream of the shunt valve 64. The supply line return piston 62 is a single direction push piston. Thus, if pressure is lost in the supply line 12, the return piston 62 loses pressure and moves the park valve 60 to its second position, automatically.

However, it is not used to move the park valve 60 to its first position -this is controlled by manual operation of the park valve 60 or the electronic options discussed above.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The invention may also broadly consist in the parts, elements, steps, examples and/or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and/or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and/or to encompass equivalents.

Claims

CLAIMS1 A trailer braking system including: an electronic control unit which is operable to send and receive signals, a spring brake control assembly which controls a spring brake actuator, the spring brake actuator having an applied state in which a brake force is applied to a wheel and a released state in which no brake force is applied to the wheel, a supply line for supplying air to the spring brake control assembly and / or the spring brake actuator, a park valve which has a first position in which the supply line is open and connects to the spring brake control assembly and / or the spring brake actuator and the spring brake actuator is controlled by the spring brake assembly, and a second position in which the supply line is closed and air is prevented from reaching the spring brake control assembly or the spring brake actuator, and a park override valve which is connected to a control port of the park valve and the park override valve is biased to a first position in which the control port is connected to an exhaust and has a second position in which the control port is connected to a supply of air, wherein the park override valve is electrically operable to its second position in response to an electronic signal.
2. A trailer braking system according to claim 1 further including a trailer reservoir for storing air.
3. A trailer braking system according to claim 1 or 2 wherein the park override valve includes a connection to the supply line.
4. A trailer braking system according to claim 3 wherein when the park override valve is in its second position, the supply line is connected to the control port of the park valve.
5. A trailer braking system according to claim 4 wherein the park override valve includes a first port which is connected to the control port of the park valve, a second port which is connected to the supply line and a third port which is the exhaust / vents to atmosphere.
6. A trailer braking system according to claim 2 wherein the park override valve includes a connection to the trailer reservoir.
7. A trailer braking system according to claim 6 wherein when the park override valve is in its second position, the trailer reservoir is connected to the control port of the park valve.
8 A trailer braking system according to claim 7 wherein the park override valve includes a first port which is connected to the control port of the park valve, a second port which is connected to the trailer reservoir and a third port which is the exhaust / vents to atmosphere.
9 A trailer braking system according to any one of the preceding claims further including a mobiliser valve which has a first position in which air in the supply line is permitted to the spring brake control assembly and a second position in which air in the supply line is prevented from reaching the spring brake assembly.
10. A trailer braking system according to claim 9 wherein the mobiliser valve is pneumatically driven via a control port.
11. A trailer braking system according to claim 10 further including an emergency override valve which is connected to the control port of the mobiliser valve and is controlled by the electronic control unit.
12 A trailer braking system according to claim 11 wherein the emergency override valve has a first position in which the supply line is connected to the spring brake control assembly and the control port of the mobiliser valve and a second position in which the trailer reservoir is connected to the spring brake control assembly and the control port of the mobiliser valve.
13. A trailer braking system according to claim 9 wherein the mobiliser valve is controllable, between its first and second positions, electrically via signals from the electronic control unit.
14. A trailer braking system according to any one of the preceding claims wherein the electronic signal that controls operation of the park override valve is generated by the electronic control unit, and optionally the electronic signal is generated on receipt of a signal from an electronic control unit on a towing vehicle.
15. A trailer braking system according to any one of the preceding claims wherein the park valve includes a release piston at its control port which is connected to the park override valve, so that the park valve is moved to its first position, when the park override valve is in its second position, by the release piston.
16. A trailer braking system according to claim 15 wherein the release piston is a single direction push piston.
17. A trailer braking system according to any one of the preceding claims further including a shunt valve which has a first condition in which the supply line is connected to the park valve and a second condition in which the trailer reservoir is connected to the park valve.
18. A trailer braking system according to any one of the preceding claims wherein a park valve sensor is provided which outputs a signal indicative of the position of the park valve 19.
A trailer braking system according to claim 18 wherein the park valve sensor is a reed switch which has two states and outputs a constant signal when the park valve is in one of its positions and no signal when the park valve is the other one of its positions.
A method of operating a trailer braking system according to any one of the preceding claims, including: the electronic control unit receiving a signal from an external source to release the park valve, and generating a signal for sending to the park override valve, actuating the park override valve to its second position and connecting a supply of air to the control port of the park valve, causing the park valve to move to its first position, controlling the spring brake control assembly to enable movement of the spring brake actuator to its released state.
21. A method of operating a trailer braking system according to claim 20 when further dependent on any one of claims 9 or 13 wherein, after actuating the park override valve, the method further includes the electronic control unit sending a mobilise signal and actuating the mobiliser valve to its first position.
22. A method of operating a trailer braking system according to claim 20 when further dependent on claim 11 or 12 wherein the mobilise signal is sent to the emergency override valve, actuating the emergency override valve to its second position.
23. A method of operating a trailer braking system according to claim 20 when further dependent on claim 13 wherein the mobilise signal is sent to the mobiliser valve and actuating it to its first position.