An hydraulic braking pressure control unit for a vehicle includes a first valve member installed at an intermediate point in a flow passage providing communication between the rear brake system of a tandem master cylinder and rear wheel cylinders, the valve member is adapted to open and close the passage, and a proportioning valve is normally urged in one direction to keep the valve member away from its seat to establish communication between the tandem master cylinder and the rear wheel cylinders. The proportioning valve is adapted, when the braking pressure is supplied from the tandem master cylinder, to reduce the braking pressure at a predetermined rate and supply the reduced braking pressure to the rear wheel cylinders. A passage is provided for communication between passages formed upstream and downstream of the proportioning valve, and a second valve member is arranged to open and close the communicating passage. The second valve member has a passage formed on its side facing the opening direction for the flow of the braking pressure for the rear brake system of the tandem master cylinder and also has a passage formed on its side facing the closing direction for the flow of the braking pressure for the front brake system of the tandem master cylinder. The second valve member is in the closed position when it is subjected to the braking pressures of both the rear and front brake systems.
A braking pressure control unit comprises a valve assembly with a valve seat (25) formed fast with the housing and a closure member (26) which latter, with the brake circuits (I, II) intact, when performing its relative movement to the valve seat (25), is kept in abutment on an extension (II) of a stepped piston (6) by a closure spring (28) supported on a malfunction piston (7). The valve (25, 26) is prevented from closing by the extension (11) at the stepped piston (6) and by a stop (29) for the closure member (26) at the malfunction piston (7). Because of the closure member (26) being in constant abutment on the extension (11) of the stepped piston (6) during its relative movement, there results a smaller stroke of the malfunction piston (7) and thus less fluid input of the braking pressure control unit and less wear of the sealing elements (22, 23) at the malfunction piston (7).
