A hydraulic dual-circuit brake system for an anti-skid system and traction control for motor vehicles including at least one driven vehicle wheel and a brake cylinder for each at least one driven vehicle wheel;
- comprising a master brake cylinder, with first and second separate brake circuit outlets, for controlling a brake pressure by brake pedal actuation;
- a brake fluid tank that communicates with the master brake cylinder;
- a four-channel hydraulic unit connected to the first and second separate brake circuit outlets of the master brake cylinder, said four-channel hydraulic unit comprises:
- first, second, third and fourth outlet channels, distributed to the first and second separate brake circuits, for connecting the first and second separate brake circuits to the wheel brake cylinders of the vehicle wheels,
- first, second, third and fourth electromagnetic control valves each having inlets, with one each electromagnetic control valve assigned to one each of said first-fourth outlet channels, for controlling a wheel-slip-dependent brake pressure in the wheel brake cylinders, each of said electromagnetic control valves communicate with one outlet channel and in pairs via one connecting line connected with a respective brake circuit outlet to the master brake cylinder,
- a return pump with first and second separate pump elements, said first and second pump elements being operative in said first and second separate brake circuits, respectively, for returning brake fluid to said first and second separate brake circuits upon brake pressure reduction in the wheel brake cylinders, said first and second separate pump elements can each be made to communicate on an inlet side, via the first, second, third, and fourth control valves, with the first, second, third, and fourth outlet channels belonging to one of the first and second brake circuits, and on an outlet side of said first and second separate pump elements to communicate via an additional connecting line, with the brake circuit outlet to the master brake cylinder associated with the brake circuit under brake pressure reduction in the wheel brake cylinders, and
- first and second low-pressure reservoirs, each assigned to the first and second brake circuit, respectively, for temporarily holding brake fluid during brake pressure reduction, each of said first and second low-pressure reservoirs is connected to the inlet of the first and second pump element respectively;
- and comprising an additional hydraulic unit for furnishing a brake supply pressure during traction control, said additional hydraulic unit has
- a precharging pump that communicates on an inlet side with the brake fluid tank and on an outlet side thereof, via a first check valve (46), with the inlet of the second pump element assigned to said second brake circuit having at least one drivel wheel, said precharging pump upon traction control feeds brake fluid into said third and fourth outlet channels each containing at least one driven wheel, and
- a valve unit (49), which upon traction control interrupts the connecting line that connects said second pump element assigned to said second brake circuit having at least one driven wheel to the master brake cylinder and connects the pump element assigned to said second brake circuit to a pressure limiting valve, and
- a shutoff element, which disconnects the second low-pressure reservoir from the precharging pump is operative in traction control, and is incorporated between the low-pressure reservoir, assigned to the second brake circuit having at least one driven wheel, and a connection point of the low-pressure reservoir to the second pump element of the return pump (27, 127), said shutoff element is embodied as a 3/2-way magnet valve, including first, second, and third controlled valve connections, the first controlled valve connection communicates with the second low-pressure reservoir, the second controlled valve connection communicates with the inlet of the second pump element, and the third controlled valve connection communicates with the brake fluid tank (18), and that the first valve connection in an unexcited basic valve position, communicates with the second valve connection and, in a reversed valve position, communicates with the third valve connection.