When a tractor is connected to a trailer and the service and emergency lines are opened, the relay emergency valve permits charging the trailer air brake reservoir to approximately the same air pressure as that in the tractor reservoirs. During normal operation of a tractor-trailer unit, the relay emergency valve functions as a relay valve and synchronizes trailer service brake air pressure and tractor service brake air pressure, as the treadle valve on the tractor is operated. The trailer brakes can also be applied independently of the tractor brakes by use of the hand control on the tractor protection valve on the tractor and the relay emergency valve on the trailer.

If a trailer is disconnected from a tractor for loading or unloading or if the trailer is separated from the tractor under emergency breakaway conditions or if the emergency line of the trailer is vented to the atmosphere by other means, the relay emergency valve applies the trailer brakes. This is automatically achieved by using the existing trailer reservoir air pressure. If the trailer is to remain parked under these conditions, the wheels should be blocked to prevent the possibility of a runaway.

If you are required to release the emergency brake application on a trailer under these conditions, the trailer reservoir drain valve can be opened or the trailer air brake system can be recharged through the trailer emergency line.

You can check the relay emergency valve by moving the tractor protection valve control lever to the EMERGENCY position, if tractor protection equipment is installed. If no tractor protection is

Figure 7-53.- Relay emergency valve.

installed, by closing the emergency line cutout valve and uncoupling the emergency brake line, the valve can be checked. Either way the trailer brakes should apply automatically. Trailer brakes should release, in the first case, when the tractor protection valve control lever is moved to the NORMAL position, and, in the second case, when the emergency line is coupled and the cutout valve is opened.

The relay emergency valve is checked for leakage by application of soapsuds with the brakes released. Check the emergency air line coupling with soapsuds to determine leakage with the valve in emergency application position. Leakage may be caused by dirt or worn parts which may be corrected by cleaning and/ or replacing the unit.

Check Valves
Check valves are located in the lines of air brake systems to prevent the loss of air should the line rupture while in operation. These are placed at the entrance of the main air tanks and prevent the loss of air should the inlet line from the compressor fail. The ball-type check valve (fig. 7-54) is typical of the type used on trailer braking systems. Check valves may be either disc or ball and double or single units. Regardless of their design, their function is the same.

Figure 7-54.- Ball-type single check valve.