| 摘要 |
In the case of a brake pressure control unit (1) for a motor vehicle hydraulic or pneumatic brake system the brakes on the rear wheels of the vehicle are acted upon by two separate brake circuits. The unit essentially comprises a pressure relief valve (10) and a floating piston pressure reducing valve (110) which is pressure-medium connected to the latter, the first brake circuit (2) being controlled by the pressure relief valve (10) and the second brake circuit (3) by the pressure reducing valve (110). The pressure relief valve comprises a differential piston (70), which is acted upon by a first flexible element (56) in the opening direction of the valve, which element is in turn supported on a clamping piston (50), the clamping piston being acted upon by the master cylinder pressure against the spring force of the first flexible element and the master cylinder pressure acting on the clamping piston being limited by a valve (40, 44) closing as a function of the vehicle deceleration. For the pressure relief valve (10) a second flexible element (94) can be provided, which acts on the differential piston (70) in the closing direction of the pressure relief valve and the spring force of which is less than the spring force of the first flexible element (56). The outlet side of the first brake circuit (2) has a pressure medium connection to a floating piston (118) of the pressure reducing valve (110) in such a way that, owing to the floating piston action of the floating piston (118), the same pressure is also applied to the associated wheel brake cylinder (Rz II) of a rear wheel of the vehicle in the second brake circuit (3) as in the case of the other wheel brake cylinder (Rz I) of the rear axle. In this way the good control characteristics of the first brake circuit (2) can also be transferred to the second brake circuit (3) by simple means. The brake pressure control unit (1) permits emergency operation in the event of failure of a brake circuit. If the second brake circuit (3) fails, the full control function of the pressure relief valve (10) of the first brake circuit (2) is retained. If the first brake circuit (2) fails, the floating piston (118) of the pressure reducing valve (110) shifts into a so-called "locking position", in which the second brake circuit (3) is switched through and unreduced pressure of the master cylinder can be fed to the associated wheel brake cylinder (Rz II). <IMAGE>
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