Brakes applied (Fig. 12.19(a)) When the brakes are applied, a signal pressure from the foot control valve (or hand control valve) reacts on the large control piston which responds by moving downwards rapidly until the centre stem of the piston closes the exhaust passage. The downwards movement of the piston pushes open the inlet valve. Air will now be admitted to the underside of the piston as it flows through to the service line and brake actuator. Movement of air from the service reservoir to the service line continues until the combined upthrust of both piston and valve springs and the air pressure balances the air signal pressure force, pushing the piston downwards. The piston now rises, closing the inlet valve so that both inlet and exhaust valves are in the lapped condition.

Brakes hold (Fig. 12.19(a and b)) A reduction in signal pressure now produces a greater force, pushing the piston upwards rather than downwards. The piston rises, closing the inlet valve, followed by the opening of the exhaust valve. The trapped air in the service line and actuator will now exhaust through the hollow valve stem to the atmosphere. The exhaustion of the service line air continues until the upward piston force balances the downward force caused by signal pressure. Both inlet and exhaust valves will subsequently close. These cycles of events are repeated the instant there is a change in signal pressure, be it an increasing or decreasing one, the valve being self-lapping under all conditions.

Brakes released (Fig. 12.19(b)) When the brakes are released, the signal pressure collapses, permitting the piston return spring to raise the piston; first closing the inlet valve, and then opening the exhaust valve. Air in the service line then escapes

Fig. 12.19 (a and b) Relay valve through the lower piston chamber and out into the atmosphere through the hollow valve stem.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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