Operation

Brakes released (Fig. 12.26(a)) When the brake pedal is released, compressed air exhausts from the supply dump valve tractor and trailer service line sensing chambers. Under these conditions, the piston spring forces the piston and exhaust valve stem down and unseats the inlet valve. Air from the tractor emergency (supply) line is therefore free to flow through the supply dump valve to the trailer's emergency (supply) line to charge the trailer's reservoir.

Service brakes applied (Fig. 12.26(b)) When the foot pedal is depressed the tractor service line output from the foot control valve and the multi-relay valve output to the trailer service line both send a pressure signal. The air pressure in both the upper and lower chambers will therefore be approximately equal. Because the piston's upper surface area is greater than its underside area and the piston spring applies a downward thrust onto the piston, the piston will be forced to move to its lowest position. This lowering of the piston closes the exhaust valve and opens the inlet valve. Air is now able to flow from the tractor emergency

(ci Front service line applied

Fig. 12.25(a-d) Multi-relay valve application

(ci Front service line applied

Fig. 12.25(a-d) Multi-relay valve application

(supply) line to the trailer's emergency line via the open inlet valve mounted in the lower part of the dump valve. As a result, when the service line pressure signal is delivered to the emergency relay valve, the emergency (supply) line passes compressed air to the brake actuators on the trailer, thereby engaging the brakes.

Failure of service line pressure (Fig. 12.26(c)) Should the trailer service line be at fault, causing the piping or coupling to leak, the air pressure in the upper trailer service line sensing chamber will be lower than that in the tractor service line sensing chamber. Consequently the piston will lift, causing the inlet valve to close so that no more compressed air passes to the trailer emergency line and the exhaust valve becomes unseated. Air trapped in the trailer emergency line will immediately discharge through the centre hollow exhaust valve stem to the atmosphere. Once the trailer emergency

Fig. 12.26 (a-c) Supply dump valve (Bendlx)

line pressure has dropped below 2 bar, the emergency relay valve inlet passage opens, permitting the compressed air stored in the trailer reservoir to discharge into the trailer brake actuators. The towing and towed vehicles are therefore braked to a standstill.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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