US3667229A

US3667229A - Device for supplying fluid under pressure

Info

Publication number: US3667229A
Application number: US884890A
Authority: US
Inventors: Arturo Cresto
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-12-18
Filing date: 1969-12-15
Publication date: 1972-06-06
Anticipated expiration: 1989-06-06
Also published as: DE1963114B2; JPS5028587B1; FR2026459A1; DE1963114C3; DE1963114A1

Abstract

A device for supplying fluid under pressure has a two-part cylinder, one part being of narrower diameter than the other, and a two-part piston slidable in the cylinder. Two pressure chambers are defined by the cylinders and the piston. Advance of the piston reduces the volume of both. One, the high pressure chamber feeds an external pressure system, and is itself fed by the other chamber through a valve system during an initial phase of the piston stroke. In a later phase the feed from the second in the first chamber ceases and instead the second chamber feeds the fluid reservoir through a control valve. Pressure-transmitting means such as a rod is arranged between the high pressure chamber and the control valve to open the control valve when the pressure in the high pressure chamber exceeds a predetermined value.

Description

United States Patent Cresto 1 June 6, 1972 [54] DEVICE FOR SUPPLYING FLUID 3,376,705 4/1968 Valpreda ..60/54.6 R UNDER PRESSURE 2,311,787 2/1943 Swift ..60/54.6 A

[ Inventor: Al'tlll'o Turin, Italy FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Ferruccio Manzini, Turin, Italy 566,623 11/1958 Canada ..60/54.6 A

[22] Ffled: 1969 Primary Examiner-Carlton R. Croyle [21 Appl. No.: 884,890 Assistant Examiner-John J. Vrablik Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak Foreign Application Priority Data 57] ABSTRACT Dec. 18. 1968 Italy ..54336 A/68 A device for Supplying fluid under pressure has a mop! cylinder, one part being of narrower diameter than the other, i and a two-part piston slidable in the cylinder. Two pressure 417/255 260 261 chambers are defined by the cylinders and the piston. Ad-

""""""" 4117/2841 vance of the piston reduces the volume of both. One, the high pressure chamber feeds an external pressure system, and is itself fed by the other chamber through a valve system during an [56] References cued initial phase of the piston stroke. In a later phase the feed from UNITED STATES PATENTS the second in the first chamber ceases and instead the second chamber feeds the fluid reservoir through a control valve. 2.031 .360 2/1936 Boughton ..60/54.6 A pressure transmitting means Such as a rod i arranged between 1672909 3/1954 the high pressure chamber and the control valve to open the 2'739'448 3/1956 control valve when the pressure in the high pressure chamber 314L303 7/1964 exceeds a predetermined value. 3,171,257 3/l965 3,232,628 2/1966 Brand ..60/54.6 A 12 Claims, 2 Drawing Figures i I l 32 i I\ URI H I\ 3a 41 4 19 5 s3 s7 58 60 w will 54 I z 81 24 A 52 2 l /i n 22 1 \/\1 \I\\ LI\ L 17 1 17 ai) :14 3 7\ 2 I 28 DEVICE FOR SUPPLYING FLUID UNDER PRESSURE This invention relates to a device for supplying fluid under pressure to an external pressure system. The device is particularly suitable for operating hydraulic brake systems in motor vehicles.

In such devices, it is generally desirable to have a first phase in the stroke of an operating member such as a pedal-controlled piston in which the pressure in the fluid supplied increases comparatively gently, and a later phase in which the increase is sharper.

The object of the invention is to provide such a device in which there is acceptably large pressure augmentation of this desirable kind during the piston stroke, the change from the low pressure phase to the high pressure phase moreover taking place progressively and smoothly.

The invention provides a device for supplying fluid under pressure to an internal pressure system, the device comprising a first cylinder connected to a fluid reservoir through a port in the cylinder wall; a second cylinder of smaller diameter than the first cylinder, opening at one end into the end wall of the first cylinder and being closed at its other end; a piston reciprocable in the cylinders and having a first section slidable in the first cylinder and a second section slidable in the second cylinder; means biasing the piston away from the closed end of the second cylinder, the piston being adapted to be moved on an advance stroke against the resistance of the biasing means; the piston forming with a portion of the first cylinder adjacent its end wall a first pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, one of the pressure chambers being a low pressure chamber and the other a high pressure chamber, the high pressure chamber having a port adapted to be connected to an external pressure system; the device including means to allow communication between the low pressure chamber and the high pressure chamber during a first phase of the advance stroke of the piston and to prevent such communication during the remainder of the advance stroke; and means to control communication between the low pressure chamber and the fluid reservoir, such control means including a duct between the low pressure chamber and the fluid reservoir and a control valve in the duct, the control valve having a closure member and a seat and means biasing the closure member towards the seat to close the duct but yieldable to allow communication when the pressure in the low pressure chamber exceeds a predetermined value; the control means further including pressure-transmitting means arranged between the duct and the high pressure chamber and adapted to exert force tending to move the closure member away from its seat when the pressure in the high pressure chamber exceeds a predetermined value.

Other features and objects of the invention will appear from the following description of an embodiment of the invention referring to the accompanying drawings, in which:

P16. 1 is a longitudinal sectional view of a fluid pressure producing device;

FIG. 2 is a curve illustrating pressure changes in fluid transmitted to an external system during operation of the device of FIG. 1.

The device of FIG. 1 includes a cylinder block 1 cast integrally with a fluid reservoir la.

The cylinder 2 of the block has a relatively large diameter and communicates with the atmosphere at an end of the cylinder (seen on the right) that is provided with a small flange 1b for attachment of a dust cover (not shown). At the other end the cylinder 2 has a larger diameter end portion 2a which is threaded to receive a threaded portion 3a of a cylinder head 3. The head 3 has a cylindrical cavity' 10 opening into the cylinder 2 at the end opposite the flange 1b. The head 3 is provided with a reduced diameter concentric tubular extension 3b projecting into the cylinder 2 with substantially no clearance. A washer 4 is provided to seal the connection between the head 3 and block 1. A threaded hole 5 is provided in an end wall 3c of the head 3 to receive a fitting (not shown) connecting the interior of the head 3 through

pipes

6 and 7 to hydraulic cylinders 8 which actuate brake shoes 9 holding friction elements 9a which are a part of the wheel brakes of a vehicle (not shown).

The cylinder 10 is coaxial with but smaller in diameter than the cylinder 2, and the free end surface of the extension 3b forms an end wall of the cylinder 2. At its closed end the cylinder 10 communicates with a still smaller diameter coaxial bore 1 l which in turn communicates with the threaded hole 5. Machined in the wall of the cylinder 10, at some distance from the free end of the tubular extension 3b, is an annular groove 12 of rectangular cross section in which seats a packing ring seal 13 that is provided with a tapered inner lip 13a whosefree edge faces towards the end wall 30 of the head 3. The configuration of the lip 13a is such that the minimum diameter of the lip is substantially smaller than the diameter of the cylinder 10.

In the cylinders 2 and 10 is slidably mounted a piston consisting of a section 14 accommodated in the cylinder 2 and a separate smaller diameter section 15 which extends with radia1 clearance 16 into the cylinder 10. The section 15 of the piston is provided with a radial flange 15a facing the section 14 and housed in the cylinder 2. v

The section 15 also has a cylindrical cavity 17 coaxial with the cylinders 2 and 10 and has a stepped free end to form a radial shoulder 18 facing the end wall 30. At its other end the cavity 17 is closed by an end wall 17a in which is drilled a smaller axial hole 19. I

A disc 20 provided with axial holes 20a is fitted in the stepped end of the cavity 17 and is forced against the radial shoulder 18 by one end of a compression spring 21, the opposite end of the spring being held and centered in the bore 11. The spring 21 is pre-loaded to maintain the section 15 of the piston against the section 14 to bias the

piston

14,15 towards a stop consisting of a circlip 22 seated in a groove in the wall of the cylinder 1 near the flange lb.

The end of the piston section 14 nearest the flange lb has a central recess 23 that houses an end of an operating rod 24 actuated in a conventional way, for instance by a brake pedal (not shown).

Along its mid-portion, the section 14 of the piston is provided with an annular groove 25 which is sufficiently broad to be in constant communication with the interior 27 of the fluid reservoir 1a through a port 26 drilled in the wall of the cylinder 2, regardless of the position of the piston section 14.

The section 14 is also provided near'the recess 23 with an annular groove 28 housing a packing ring seal 29 with an outer sealing lip 29a which is constantly. compressed against the wall of the cylinder to seal fluid within the cylinder.

The other end of the piston section 14 includes a head 24 which extends inwardly to a rectangular cross section peripheral groove defined by radial surfaces 30 and 3] and a cylindrical surface 32. This groove houses a packing ring seal 33 having a substantially rectangular cross section and a slightly outwardly convex contour to seal against the wall of the cylinder 2. The inner diameter of this seal is larger than the diameter of the cylindrical surface 32 and its axial dimension is smaller than the axial distance betweenthe

surfaces

30 and 31.

The head 34 is also provided with a set of axial passages 35 in communication with axial passages 36 machined in the flange 15a of the piston section 15.

Inward of the seal 33 on the piston body 14 is a shoulder 37 that is slightly smaller than the diameter of the cylinder 2 so as to provide an annular passage 38 between the inside of the groove housing the seal 33 and the annular groove 25.

In addition to communicating with the cylinder 2 through the passage 26, the interior 27 of the fluid reservoir 1a can also communicate in a conventionalway with the cylinder 2 through a small compensating orifice 39.

The axial passages 36 are in communication with the hole 19 through radial passages 40, and communication between the hole 19 and an internal cavity 41 of the piston section 15 is controlled by a pressure valve 42. This valve consists of a valve body 43 which is slidably mounted in the cavity 41 and is provided with axial peripheral passages 44. It carries on its side facing the end wall 17a a mushroom-shaped extension whose head 45 holds in place a packing ring seal 46 with an extension 46a which projects beyond the head 45 in the direction of the wall 17a to prevent communication between the hole 19 and the cavity 41. The valve body 43 and its integral extension have a concentric hole 47 which houses a pressure-transmitting member in the form of a sliding rod 48. The hole 47 is stepped towards its end facing the disc 20 to form seats for a sealing element such as an O-ring of a low friction coefficient material, an annular washer 50 and a circlip 51 on which seats one of a light tapered coil spring 52 whose other end is seated, on the disc 20 and which biases the valve 42 towards'the closed position. The rod 48 extends into the cavity 41 where it terminates in an enlarged head 48a.

The section 14 of the piston is provided with a central axial hole 53 which opens into an end face 54 of the head 34 and which communicates with the hole 19. Near the radial face 54 the hole 53 has a suitable contour to hold a sleeve 55, preferably of rubber, which acts as a valve seat for a control valve whose closure member is a ball 56, preferably of metal. A compression spring 57 housed in the hole 53 biases the ball 56 against the sleeve 55 to prevent communication therethrough. The portion of the hole 53 that houses the compression spring 57 is connected to the annular groove 25 through a blind hole 58-and a calibrated radial hole 59 which restricts the flow of fluid through the duct formed by the

cavities

19, 53, 58, 59.

The length of the rod 48 extending from the rod head 48a to the ball 56 is greater than the distance between the ball 56 and the face of the washer 50 facing the disc 20 when the parts are in-an inoperative position as shown in FIG. 1.

The diameter of the rod 48 in turn is smaller than the minimum diameter of the contact area between the ball 56 and the sleeve 55.

A so-called low pressure chamber 63 of annular shape is formed by the free end radial surface of the extension 3b which acts as the end wall of the cylinder 2, and by the cylindrical surface of the piston section 15 and by the radial surface of the flange 15a facing the extension 3b. The volume of this chamber naturally varies with the position of the piston in its advance strokes (towards the end wall 30) and its return strokes (away from the wall 30). Q A so-called high pressure chamber 64 is formed by the cylinder adjacent its end wall 30, this chamber extending to the right in FIG. 1 to include the cavity 41 in the piston section 15. The volume of this chamber also changes with movement of the piston in its strokes.

The operation of the device is as follows.

With the device in its inoperative position, the interior 27 of the reservoir la is in communication with the low pressure chamber 63 through the composite passage formed by the hole 26, the peripheral passage 38, a gap 61 between the seal 33 and the annular surface 31, an annular passage 62 between the surface of the seal 33 and the cylindrical surface 32, and the

axial passages

35 and 36. The low pressure chamber 63 is also in communication through the radial clearance 16 with thehigh pressure chamber 64 which is connected to the wheel cylinders 8 through the composite passage formed by the threaded hole 5 and the

pipes

6 and 7. The pressure valve 42 and control valve 56 are in their closed positions under the action of their springs 52 and 57. 7

When the operating rod 24 is moved in the direction of the arrow 60, the piston l4, starts to advance in the direction of chamber 64. Under these conditions there occurs first a relative movement between the seal 33 and the head 34 of the piston section 14, such movement lasting until the seal abuts the surface 31. This will generally occur even before the section 15 of the piston has slipped into the conical lip 13a of the seal 13, and blocks communication between the reservoir 27 and the low pressure chamber 63. Consequently, further advance of the piston will result in a displacement of fluid from the low pressure chamber 63, whose volume is meanwhile diminishing, to the high pressure chamber 64 through the radial clearance 16 and through the valve pressure 42, which the fluid pressure will open against the action of the spring 52.

When the free end of the section 15 of the piston is fully inserted in the inner bore of the lip 13a, fluid flow between chamber 63 and chamber 64 through the radial clearance 16 will cease but flow will continue through the valve 42 until the pressure of the fluid has reached a value such that the force will be applied to the ball 56 of the control valve capable of overcoming the load of the spring 57 From this instant on, the fluid pressure in the annular chamber 63 will not be able to increase substantially, and the valve 42 will close to sever completely communication between the chambers 63 and 64.

Pressure changes in the chambers 63* and 64 from initial operation of the unit to the instant mentioned above are represented by the line O-l in the curve of FIG. 2 where increasing force on the operating rod 24 is shown along the abscissa axis and pressures in the low and high pressure chambers are shown along the ordinates axis. a a

The difference in pressure between chambers 64 and 63 causes the rod 48 to move against the ball 56.

After a further slight movement of the piston under the increasing force applied by the rod 24, a slight pressure increase will result in the high pressure chamber 64 as compared to the pressure that would result in this chamber if it were isolated from the chamber 63. This increase in pressure is represented by the line l-ll on the FIG. 2 curverstarting from a given force applied to the piston, the control valve 56 starts to open under the combined effect of the pressure existing in the chamber 63 and the force applied by the rod 48. Increasing the force applied to the piston will result in a smooth, progressive opening of the valve 56 until the valve reaches its fully open position as determined by the movement of the rod 48 to its fully extended position in the direction of the ball 56.

The resulting increase in fluid pressure in the high pressure chamber 64 is represented by the line Illll in FIG. 2. From this point on, pressure augmentation in the chamber 64 is proportional to the force increase on the piston whereas the fluid pressure in the low pressure chamber 63 is maintained-substantially at an atmospheric level as shown by the FIG. 2

curve.

The calibrated restriction 59 provided in the duct from the low pressure chamber 63 to the fluid reservoir 27 ensures a regular fluid delivery to the high pressure chamber and hence to the wheel cylinders in cases of pressure surges such as a slam brake application, Without the restriction 59, there would be a peak of fluid pressure which would open the valve 56 prematurely and cause fluid in the low pressure chamber to flow to the reservoir before the required amount had been delivered to the wheel cylinders.

When braking ceases, the

piston

14,15 will start to return to its retracted position. The packing ring seal 33 will automatically move against its shoulder 34, thereby providing a large passage for a free return flow of fluid from the reservoir 27 to the low pressure chamber 63, and the high pressure chamber 64 will again communicate with the low pressure chamber 63 and hence with the fluid reservoir when the piston reaches its fully retracted inoperative position.

Naturally, the details of the pressure producing device of the invention may be different from those of the embodiment described above.

For instance, a different configuration may be used for the means controlling communication between the fluid reservoir 27 and the low pressure chamber 63, and between the low and the high pressure chambers 63 and 64. The low pressure chamber may be formed by the space adjacent the end wall 3c of the cylinder head 3 then being formed the annular space between the periphery of the piston section 15, the piston flange 15a and the free end surface of the tubular portion 3b of the head 3 of the cylinder, provided the arrangement of the various valves and porting is suitably changed. Also the various seals used may be different from those illustrated. The valves may be mounted in fixed housings in the walls of the cylinders instead of being mounted within the piston.

What I claim is:

1. A device for supplying fluid under pressure to an external pressure system, the device comprising a first cylinder connected to a fluid reservoir through a port in the cylinder wall, one end of the cylinder having an end wall; a second cylinder of smaller diameter than the first cylinder, opening at one end into the end wall of the first cylinder and being closed at its other end; a piston reciprocable in the cylinders and having a first section slidable in the first cylinder and a second section slidable in the second cylinder; means biasing the piston away from the closed end of the second cylinder, the piston being adapted to be moved on an advance stroke against the resistance of the biasing means; the piston forming with a portion of the first cylinder adjacent its end wall a first pressure chamber, a portion of the second cylinder'adjacent its closed end forming a second pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, one of the pressure chambers being a low pressure chamber and the other a high pressure chamber, the high pressure chamber having a port adapted to be connected to an external pressure system; the device including means to allow communication between the low pressure chamber and the high pressure chamber during a first phase of the advance stroke of the piston and to prevent such communication during the remainder of the advance stroke; and means to control communication between the low pressure chamber and the fluid reservoir, such control means including a duct between the low pressure chamber and the fluid reservoir and a control valve in the duct, the control valve having a closure member and a seat and means biasing the closure member towards the seat to close the duct but yieldable to allow communication when the pressure in the low pressure chamber exceeds a predetermined value, the control means further including pressure-transmitting means arranged between the duct and the high pressure chamber and adapted to exert force tending to move the closure member away from its seat when the pressure in the high pressure chamber exceeds a predetermined value, said control means further including a passage extending between the low pressure chamber and the reservoir, said, said passage being defined by an annular groove in the first section of the piston and a sealing ring located in the groove and bearing sealingly against the cylinder surface, the sealing ring having axial and radially inner clearance with respect to the groove, whereby upon movement of said piston during an advance stroke said sealing ring is engaged by a wall of said groove to close said passage and upon a return stroke will be separated immediately from the wall of said groove to open said passage.

2. The device of claim 1 in which the sealing ring is of an elastic material.

3. A device for supplying fluid under pressure to an external pressure system, the device comprising a first cylinder connected to a fluid reservoir through a port in the cylinder wall, one end of the cylinder having an end wall; a second cylinder of smaller diameter than the first cylinder, opening at one end into the end wall of the first cylinder and being closed at its other end; a piston reciprocable in the cylinders and having a first section slidable in the first cylinder and a second section slidable in the second cylinder; means biasing the piston away from the closed end of the second cylinder, the piston being adapted to be moved on an advance stroke against the resistance of the biasing means; the piston forming with a portion of the first cylinder adjacent its end wall a first pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, one of the pressure chambers being a low pressure chamber and the other a high pressure chamber, the high pressure chamber having a port adapted to be connected to an external pressure system; the

device including means to allow communication between the low pressure chamber and the high pressure chamber during a first phase of the advance stroke of the piston and to prevent such communication during the remainder of the advance stroke; and means to control communication between the low pressure chamber and the fluid reservoir, such control means including a duct between the low pressure chamber and the fluid reservoir and a control valve in the duct, the control valve comprising resilient seat means having a circular aperture therethrough, closure means extending into said aperture in engagement with the periphery of said circular aperture and means biasing the closure member toward the resilient seat to compress the periphery of the circular aperture and close the duct whereby the closure member will yield to allow communication between the closure member and the compressed resilient seat when the pressure in the low pressure chamber exceeds a predetermined value; the control means further including pressure-transmitting means arranged between the duct and the high pressure chamber and adapted to exert force tending to move the closure member away from its seat when the pressure in the high pressure chamber exceeds a predetermined value.

4. The device of claim 3 in which the duct includes a restriction to brake fluid flow during pressure surges.

5. The device of claim 3 in which the second section of the piston has radial clearance in thesecond cylinder, and including seal means located in a groove in the second cylinder and adapted to seal against the second section of the piston, the seal means being located between the free end of the second section of the piston and the closed end of the second cylinder when the piston is at the start of its advance stroke.

6. The device of claim 3 including stop means in the first cylinder to limit the return stroke of the piston.

7. The device of claim 3 in which the first cylinder communicates with the atmosphere at its end opposite the end having the end wall.

8. The device of claim 3 in which the pressure-transmitting means is a rod arranged slidably between the duct and the high pressure chamber with one end adapted to contact the closure member and the second end exposed in the high pressure chamber.

9. The device of claim 8 in which the duct includes a passage opening into the high pressure chamber on the same side of the control valve as is connected to the low pressure chamber, the passage being controlled by a pressure valve having a closure member and biasing means biasing the closure member to close the passage but yieldable when the pressure in the low pressure chamber exceeds that in the high pressure chamber by a predetermined value, the pressure-transmitting rod being a sliding fit in a passage extending through the closure member of the pressure valve.

10. The device of claim 3 in which the control means includes a passage extending between the low pressure chamber and the reservoir and means to close the passage during the advance stroke of the piston and to open it during the return stroke of the piston.

11. The device of claim 10 in which the passage includes an annular groove in the first section of the piston and a sealing ring located in the groove and bearing sealingly against the cylinder surface, the sealing ring having axial and radially inner clearance with respect to the groove, the radial edge of the groove on the side away from the low pressure chamber being in communication with the reservoir.

12. The device of claim 11 in which the sealing ring is of an elastic material.

Claims

1. A device for supplying fluid under pressure to an external pressure system, the device comprising a first cylinder connected to a fluid reservoir through a port in the cylinder wall, one end of the cylinder having an end wall; a second cylinder of smaller diameter than the first cylinder, opening at one end into the end wall of the first cylinder and being closed at its other end; a piston reciprocable in the cylinders and having a first section slidable in the first cylinder and a second section slidable in the second cylinder; means biasing the piston away from the closed end of the second cylinder, the piston being adapted to be moved on an advance stroke against the resistance of the biasing means; the piston forming with a portion of the first cylinder adjacent its end wall a first pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, one of the pressure chambers being a low pressure chamber and the other a high pressure chamber, the high pressure chamber having a port adapted to be connected to an external pressure system; the device including means to allow communicatioN between the low pressure chamber and the high pressure chamber during a first phase of the advance stroke of the piston and to prevent such communication during the remainder of the advance stroke; and means to control communication between the low pressure chamber and the fluid reservoir, such control means including a duct between the low pressure chamber and the fluid reservoir and a control valve in the duct, the control valve having a closure member and a seat and means biasing the closure member towards the seat to close the duct but yieldable to allow communication when the pressure in the low pressure chamber exceeds a predetermined value, the control means further including pressure-transmitting means arranged between the duct and the high pressure chamber and adapted to exert force tending to move the closure member away from its seat when the pressure in the high pressure chamber exceeds a predetermined value, said control means further including a passage extending between the low pressure chamber and the reservoir, said, said passage being defined by an annular groove in the first section of the piston and a sealing ring located in the groove and bearing sealingly against the cylinder surface, the sealing ring having axial and radially inner clearance with respect to the groove, whereby upon movement of said piston during an advance stroke said sealing ring is engaged by a wall of said groove to close said passage and upon a return stroke will be separated immediately from the wall of said groove to open said passage.

2. The device of claim 1 in which the sealing ring is of an elastic material.

3. A device for supplying fluid under pressure to an external pressure system, the device comprising a first cylinder connected to a fluid reservoir through a port in the cylinder wall, one end of the cylinder having an end wall; a second cylinder of smaller diameter than the first cylinder, opening at one end into the end wall of the first cylinder and being closed at its other end; a piston reciprocable in the cylinders and having a first section slidable in the first cylinder and a second section slidable in the second cylinder; means biasing the piston away from the closed end of the second cylinder, the piston being adapted to be moved on an advance stroke against the resistance of the biasing means; the piston forming with a portion of the first cylinder adjacent its end wall a first pressure chamber, a portion of the second cylinder adjacent its closed end forming a second pressure chamber, one of the pressure chambers being a low pressure chamber and the other a high pressure chamber, the high pressure chamber having a port adapted to be connected to an external pressure system; the device including means to allow communication between the low pressure chamber and the high pressure chamber during a first phase of the advance stroke of the piston and to prevent such communication during the remainder of the advance stroke; and means to control communication between the low pressure chamber and the fluid reservoir, such control means including a duct between the low pressure chamber and the fluid reservoir and a control valve in the duct, the control valve comprising resilient seat means having a circular aperture therethrough, closure means extending into said aperture in engagement with the periphery of said circular aperture and means biasing the closure member toward the resilient seat to compress the periphery of the circular aperture and close the duct whereby the closure member will yield to allow communication between the closure member and the compressed resilient seat when the pressure in the low pressure chamber exceeds a predetermined value; the control means further including pressure-transmitting means arranged between the duct and the high pressure chamber and adapted to exert force tending to move the closure member away from its seat when the pressure in the high pressure chamber exceeds a predetermined value.

5. The device of claim 3 in which the second section of the piston has radial clearance in the second cylinder, and including seal means located in a groove in the second cylinder and adapted to seal against the second section of the piston, the seal means being located between the free end of the second section of the piston and the closed end of the second cylinder when the piston is at the start of its advance stroke.

12. The device of claim 11 in which the sealing ring is of an elastic material.