Brakes released (Fig. 12.25(a)) If the brakes are released, all three relay pistons will rise to their uppermost positions due to the return spring upthrust. Consequently, the inlet valve closes and the exhaust valve will be unseated. This ensures that the trailer brake actuators are cleared of compressed air, so releasing the brakes.

Secondary line brake application (Fig. 12.25(b)) Applying the hand control valve handle sends a pressure signal to the lower relay piston (3). The lower relay piston will move downwards, initially closing the exhaust valve and then opening the inlet valve. A pressure signal will then pass from the trailer reservoir mounted on the tractor to the upper part of the trailer's emergency relay valve. As a result, air pressure from the supply line (red) now flows to the trailer brake actuators.

Service line brake application (Fig. 12.25(c and d)) When the foot control valve is depressed a signal pressure from the both halves of the foot valve is transmitted to the upper (1) and middle (2) relay valve pistons. Both relay pistons react immediately by moving down until the three relay pistons are pressed together. Further downward movement will close the exhaust valve and open the inlet valve. Air from the trailer reservoir mounted on the tractor will now pass to the emergency relay valve, permitting air from the supply line (red) to pass directly to the trailer brake actuators via the now opened passage passing through the emergency valve.

Should half of the dual foot valve service line circuit develop a fault, the other half service line circuit will still be effective and be able to operate the multi-relay valve.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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