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.