BY HANDYMAN
Page 53
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ATHOUGH I have already covered the relay valve as a separate Unit, positioned on the tractor to give balanced braking. a further service is called for. The relay valve is again required, but this time to work in conjunction with another valve in order to deal with any damage or failure in the supply or control from the tractor, the intention being to arrange that the trailer brakes will automatically be applied. Therefore, on the two-line trailer, it is normal practice to mount a combined relay and emergency valve in the service and emergency lines, and in turn this feeds into the trailer brake supply lines.
This combination valve can be described as follows: – a normal relay valve as described in Part 6, less the bolted-on adapter, the place of the normal adapter being taken by the body of the emergency valve on the Clayton-Dewandre model, and using the same bolts.
Again there is a valve body and a cover and, starting from the base of the valve, there is the cap nut, coil spring and -the pressure-regulating valve. Retained by the cover is the emergency diaphragm, which is spring loaded, then the lower emergency valve and valve stem to the upper emergency valve, again very similar in layout to the relay valve.
The spring-loaded regulating valve is set to operate at around 70 lb. pressure, and in the body of the valve there is a non-return valve retained by a cap nut.
An important feature in the emergency line inlet port is a metal filter intended to stop the entry of dirt or abrasive material. I have already shown how the air enters the relay valve and is held at the supply valve. Compressed air then passes along the emergency line into the relay emergency valve via the filter and lifting the non-return valve, then over the top of the emergency diaphragm and on to charge the trailer reservoir.
During this time, air has been passing below the emergency diaphragm and into the space above the pressure-regulating diaphragm, and air will continue to flow until a pressure of 70 lb. is reached above the regulating diaphragm. At this point, with 70 lb. above the regulating diaphragm, the regulating valve spring is overcome, the diaphragm moving down and opening the