BC Transit – Air Brake Course

L-75 Off Highway Combination with Schedule “X” Trailer (continued)
When air pressure in the main reservoir system is above the setting of the low pressure governor (approx.
50 psi) the exhaust ports are held closed and the trailer emergency line can be charged normally. If main
reservoir pressure drops below the governor setting, the governor shuts off and exhausts air from the
control port of the dump valve. The exhaust ports on the dump valve open, venting the emergency (supply)
line to the trailer. This will cause the trailer brakes to “dynamite” and the trailer air supply dash valve to
close, shutting off further air supply to the trailer.
Low press. Gov.
Dump Valve
Trailer supply
Supply line gauge
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