750iL Vehicles:
The electric Pre-boost pump delivers a brake fluid pressure of 7-15 bar to the isolated chamber of the charge piston . The pre-boost pump incorporates an integral pressure relief valve which opens at 15 bar.
From the isolated chamber the fluid returns to the reservoir through a restriction. The restriction causes pressure to build in the chamber which pushes the pistons outward. This immediately acts on two valves which block the normal braking circuits that flow through the charge piston ports.
The pressure in the isolated chamber continues to push the pistons outward delivering the required charge pressure.
For ABS and CPT operation, the pump systems are not switched on, only during ASC/DSC regulation requiring hydraulic intervention are the pre-boost pump or the charge pump switched on. The balance of the hydraulic system functions operate in the same familiar manor of all previous systems. The individual brake circuits can be isolated as needed to restore lateral locating forces through pressure build, hold and release phases of operation. During an ASC hydraulic intervention requiring only rear wheel brake application, the inlet valves for the front wheels are closed preventing any pressure influence from the charge pump systems.