The present invention relates to an anti-lock hydraulic brake system with at least one wheel brake and an auxiliary pressure source, wherein an inlet valve is inserted into the line between the auxiliary pressure source and the wheel brake and the wheel brake is connected to a return line via an outlet valve, wherein a differential pressure limiter is disposed within the line between the auxiliary pressure source and the inlet valve, the closure member of which is acted upon by both the force of a valve spring and the pressure in the wheel brake in a manner opening the line.
A brake system of this type is known from German published patent application 38 42 699.1. The latter describes a dual-circuit brake system with a tandem master cylinder and a pump for each brake circuit. The wheel brake cylinders each communicate via a respective inlet valve with the pressure line leading to the pump and, also with the brake line leading to the master brake cylinder. Each brake cylinder communicates via a respective outlet valve with the return line to the supply reservoir. Each inlet valve is normally open, while the outlet valves are normally closed.
The rotational behavior of the wheels is constantly monitored during a braking operation. In the event of an imminent locked condition of any one of the wheels, the drive of the pump is activated, and the inlet and outlet valves of the imminently locking wheel controlled such as to develop an amount of pressure in the wheel brakes which correlates with an optimal slip value of the wheel. To decrease the pressure, the outlet valve is opened and the inlet valve closed. Both valves are closed to retain the pressure, whilst for the pressure increase the inlet valve is open and the outlet valve is closed.
The inlet and the outlet valves are electromagnetically operated valves having a closure member which is seated on a valve seat.
Starting from a brake fluid pressure which is in excess of the pressure which corresponds to the force of the valve spring, a differential pressure limiter between the auxiliary pressure source and the inlet valve causes a defined constant pressure gradient to prevail on the inlet valve which is determined by the valve spring. This pressure gradient can be adjusted to 25 bar for example, and hence is considerably less than the pressure gradient which usually occurs when an emergency stop anti-lock controlled braking operation is performed by the brake system.
However, it has been discovered that dynamic actions in the return line and in the differential pressure limiter can cause the pressure in front of the inlet valve to build up to a level higher than what is desired.
Hence the present invention has for its object to develop measures ensuring that the pressure gradient at the inlet valve remains constant under all circumstances.