Notes On Externally Invisible Brake Fluid Loss
Where brake fluid loss is not visible, it should be checked whether brake fluid has entered the brake booster through a leaking secondary seal in the tandem master brake cylinder: In this case the tandem master brake cylinder has to be removed and any ingressed brake fluid drained off and emptied into a measuring cup.
If there are more than 100 cm3 of brake fluid in the brake booster, the brake booster must be replaced.
The power diaphragm in the brake booster is resistant to brake fluid, but not the reaction disk and the poppet valve in the master control unit. Brake fluid should therefore only be drawn off when the brake booster is installed. Where there is less than 100 cm3 brake fluid in the brake booster for an installed brake booster, brake fluid will be unable to reach the reaction disk or the poppet valve.