The present invention relates to a hydraulic brake system with anti-lock control. Such brake systems include a pedal-operated pressure generator, normally a tandem master cylinder actuated by a booster-aided brake pedal. The master cylinder is connected to wheel brakes via at least one brake line which includes an electro-magnetically operated, normally open two-way, two-position valve. For pressure relief during anti-lock control operation, each of the wheel brakes is also connected to a low-pressure accumulator or reservoir via a pressure relief line. In the pressure relief line, an outlet valve is inserted which is a normally closed, electro-magnetically operated two-way, two-position valve. To return the fluid released from the wheel brakes into the brake circuit, a pump is connected with its suction side to the low-pressure accumulator or the fluid reservoir respectively, and with its pressure side through a pressure line to the brake line.
After assembling such a brake system, the entire circuitry has to be evacuated and filled with brake fluid. In order to enable a non-powered evacuation and filling of such a brake system, a check valve with a low opening pressure was placed to connect the pressure relief line to the pressure line of the pump. The check valve would open in the direction enabling a pressure flow from the relief line to the pressure line, thereby permitting the evacuation of the pressure relief line during evacuation of the pressure line. Vacuum will be kept in the pressure relief line after filling the system with brake fluid. This area will be filled with brake fluid at the end of testing the brake system during activation of the anti-lock control, thus forcing brake fluid into the pressure relief line while the normally closed valves are energized and the pump is running. While this constellation worked well for the purpose of evacuation and fill, however, it caused vacuum trapping in the return circuitry, which is the pressure relief line, after installation of the hydraulic circuitry including a valve block into a vehicle. The biggest restrictions in the brake line are the normally open electro-magnetically operated valves. When a driver releases the brake pedal, in brake systems according to the prior art, vacuum can be created between the normally open valves of the valve block and the tandem master cylinder since pressure energy between the wheel brakes and the normally open valves changes to kinetic energy at the location of the normally open valves. When this vacuum creates a sufficient pressure difference between the brake line and the pressure relief line, the bleed valve opens and traps vacuum in the pressure relief line. After a while, the vacuum will be replaced by air due to small leakage in the pressure relief line of the hydraulic circuitry, and air accumulates after a certain number of brake operations. The result is a long brake pedal stroke or a reduced braking force during electronic control of the brake operation such as anti-lock control, traction control, or driving stability control during cornering. By raising the opening pressure of the bleed valve, not as much vacuum can be trapped, however, this would result in an unsatisfactory evacuation operation.
The object of the current invention therefore is to create a brake system which enables a non-powered evacuation and filling of the circuitry on the one hand and which prevents the trapping of vacuum on the other hand.
Another object of the invention is to create a low-cost solution to the above-mentioned problem without adding excessive additional equipment.
These objectives are achieved by a brake system of the above-mentioned kind where a bypass line connects the pressure relief line with the wheel brake. This bypass line includes a bleed valve opening from the pressure relief line to the wheel brake. A possible point for connecting the bypass line including the bleed valve to the brake line is any point in direct connection with the brake line. That means if the bypass line is connected to the pressure line of the pump without any other hydraulic element in-between, that would be equivalent to connecting the bypass line directly to the brake line. This solution provides a reliable vacuum compensation during pedal releases while having a low opening pressure for the bleed valve in order to evacuate the pressure relief line properly.