Shield operator from flying chips. Prevent drum expansion. B. Prevent air from entering the system. Residual pressure would cause dragging of brakes. Residual pressure would delay front brake application.
Disc brakes use a proportioning valve.
The owner of an American passenger car complains of a pull to the right when he or she steps on the brakes. Mechanic B says the trouble could be a faulty metering valve or proportioning valve. The disc brake reservoir portion of a dual master cylinder is low in fluid. Mechanic A says the brake pads may be worn. Mechanic B says the power brake booster may be faulty.