1,270,098. Brake actuators. AUTOMOTIVE PRODUCTS CO. Ltd. 29 Aug., 1969 [23 Sept., 1968], No. 45125/68. Heading F2F. A brake actuating device comprises an output member 34 operable to apply braking means 38 on a wheel of a vehicle by fluid pressure acting on a movable wall 12 and by springs 23, means normally holding the springs in a loaded condition and releasable to permit operation of the output member to apply the braking means by the springs, a first control means to apply fluid pressure to the wall 12 to operate the member 34, and a second control means to release the spring holding means. The arrangement is such that operation of the first control means to apply fluid pressure to the wall 12 when the spring holding means has been released by the second control means, is accompanied by the application of a force to oppose the action of the springs on the member 34. As shown, the flexible diaphragm 12 defines a chamber 13, and a chamber 18 is formed by a piston 14 between which and a wall member 22 the springs 23 are arranged, the axial separation of the piston and wall member being limited by studs 27 and blocks 19 being provided for abutment therewith of the wall member. The diaphragm carries a push rod 32 which is engageable with the push rod 34, and a valve 41 mounted on the housing 10 controls admission and release of air pressure to the chamber 18. As shown in Fig. 4, an air compressor 65 driven by the vehicle engine supplies two compressed air reservoirs 66, 67, the reservoir 66 being connected through a driver's control valve 68 and a quick release valve 69 to the chambers 13 of the brake actuating devices 10, the valve 69 normally permitting air in the chambers to escape to exhaust but is actuated by air pressure passing through the valve 68 to admit air to the chambers and to seal off the exhaust connection; a valve 41 is provided on only one of the actuators and controls the air pressure in both chambers 18. The air inlet port 54 of the valve 41 is connected to the reservoir 67 and the exhaust port 55 is connected to the outlet of the valve 69 leading to one of the chambers 13. A lever 61 for actuating the valve 41 is connected to a hand control lever 72 which, as shown, is in a middle position, the valve head 51 of the valve 41 being seated on the seating ring 47 by spring 52 and the ring being spaced from the rim of the plunger 44, air pressure from reservoir 67 acting in the chambers 18 on the piston 14 to compress the springs 23. Movement of lever 72 to the right as seen in Fig. 4 moves, via lever 61, stem 50 to cause ring 47 to engage the rim of plunger 44 and close inlet port 54 from chamber 18 and then the valve head 51 away from ring 47 to allow air to exhaust from chamber 18 through valve 69, the springs 23 moving piston 14 and push rod 34 to apply the brakes; the lever 62 moves clockwise about pivot 63 pushing plunger 44 and seating ring 47 inwardly to increase the spacing between the ring and valve head 51. Release of the brakes is effected by moving lever 72 to the left so that the valve head 51 moves into engagement with the ring 47 which moves away from plunger 44 whereby air pressure from reservoir 67 again enters chamber 18 to act on piston 14 and compress springs 23. If due to leakage, air pressure is lost from reservoir 66 any attempt to apply the brakes by actuating the control valve 68 will not result in air pressure acting on the diaphragm 12. If pressure is maintained in the reservoir 67 the brakes are held off by pressure in that reservoir acting on the piston 14 but complete failure of the air system or a leak affecting the whole system allows the springs 23 to move the piston 14 and apply the brakes, such movement being transmitted through lever 62 to the plunger 44, actuating valve 41 to close reservoir 67 and connect the chamber 18 to exhaust port 55. Since the latter is connected to atmosphere through valve 69 any air pressure on diaphragms 12 to apply the brakes is transmitted also to the valve 41 and if the control valve 68 is actuated to apply pressure to the diaphragm 12 whilst the brakes are applied by the springs 23 or if the lever 72 is moved to the "park" position whilst pressure is being applied to the diaphragms, this air pressure acts on piston 14 to reduce the effective load applied by springs 23 to push rod 34 and the latter will not be subjected to the full load of both the springs and pressure on the diaphragm. In the event of application of the brakes due to complete failure of the air pressure system, the brakes are released by reducing the length of the push rod 34 which is formed of the screw-connected parts. In a modification, the device is hydraulically operated and comprises two pistons in tandem, the spring means for operating the device upon failure of the liquid pressure system consisting of a stack of Belleville washers.