553 Brake Systems Technician Handbook Piston Leakage If the primary circuit looses pressure: • The brake pedal moves with little resistance until the primary piston contacts the secondary piston. • The primary piston physically moves the secondary piston to the left. • Pressure builds in the secondary chamber and activates the secondary circuit. If the secondary circuit looses pressure: • Initially, the brake pedal moves with little resistance. • Hydraulic pressure in the primary chamber moves the secondary piston to the left until it contacts the cylinder wall. • Once the secondary piston contacts the wall, pressure builds in the primary chamber and activates the primary circuit.
In both cases the pedal travels much further than during normal braking and the overall brake force is half the normal amount. Technical Training 15