A first piston is displaceable in a housing and has a first piston portion having a larger effective surface and movable in one direction. A second piston portion normally disengaged from the first piston portion has a smaller effective surface and is moveable in an opposite direction into operative engagement with the first piston portion. The second piston is spaced from and defines with the first piston portion a chamber. A first inlet communicates with this chamber and is at the pressure prevailing in a first hydraulic brake circuit. A second inlet communicates with a smaller effective surface of the second piston portion and is at the pressure prevailing in a second hydraulic brake circuit. A biasing arrangement biases the second piston portion in the aforementioned one direction until the pressure upon the smaller effective surface exceeds the biasing force so that the second piston portion moves in the opposite direction into operative engagement with the first piston portion, with the result that the force acting upon the first piston portion urging the same in the one direction is reduced to the differential of the forces acting upon the effective surfaces of the two piston portions. A third inlet is subject to the pressure prevailing in a trailer brake circuit, and a third feed-back piston is connected with the second piston to resist movement thereof in the opposite direction and has a reaction surface exposed to the pressure of the third inlet and so dimensioned that the force acting upon it is substantially equal to the aforementioned differential.
A dual brake valve assembly for use in fluid pressure-operated vehicle braking systems embodying two independent braking circuits comprises a housing provided with sets of primary and secondary ports associated respectively with the two braking circuits and each consisting of a supply port for connection to a source of fluid under pressure and a delivery port for connection to a brake actuator, an exhaust, and two co-axial primary and secondary valves which are operative, when actuated, to isolate the delivery ports from the exhaust and connect them to the respective supply ports. A primary piston operates the primary valve directly and acts through the primary valve and a secondary piston to operate the secondary valve. The secondary piston is provided with passages through which the respective supply and delivery ports of each set are placed in communication when the respective primary and secondary valves are operated.
A relay valve device which is actuated by the presence of a control signal pressure to a higher pressure supply into a consumer circuit than when the control signal pressure is not present. The relay valve device includes a first control surface and a second control surface, and includes a shut-off device disposed in the feed line of the control pressure to the second control surface. The shut-off device is opened when the control signal pressure is present and is closed when the control signal pressure is not present, and also causes the second control surface to be connected with atmosphere.
A supply dump valve comprises a piston (1) of substantial differential outline adapted to work in a stepped bore (2) in a housing (3). Actuation of a park brake causes the piston (1) to be exercised in response to operation of the park brake.
