740i/iL & 540iA Vehicles:
When activated, the Charge Pump delivers a brake pressure of 10 - 15 bar to the front axle circuit of the hydraulic unit and into the front axle circuit pressure chamber of the master cylinder. The master cylinder piston for the front axle circuit contains a notched restrictor forward of the central valve which allows excess fluid to return to the reservoir under a restriction.
The pressurized fluid also acts on the rear axle circuit of the master cylinder. This provides the hydraulic charge to the rear axle circuit of the hydraulic unit as needed.