BC Transit – Air Brake Course

Low Air Warning Device & Safety Valve
The low air warning device is a simple spring loaded electrical switch that triggers a
warning in the driver’s compartment. This device must activate before the reservoir air
pressure drops to 60 psi. The warning can be either audible or visual. This device is
checked during the pre-trip inspection. Some systems, usually trucking applications,
use a “wig-wag” This is a metal flag that drops from above the driver’s windshield into
his/her line of sight.
The safety valve is fitted to the wet tank. Its purpose is to release excess pressure should
the governor fail to unload the compressor due to a governor failure or a problem with the
unloader mechanism in the compressor head. The safety valve should release when
pressure reaches 140 to 150 psi.
Low air warn.
safety valve
“Wig-Wag”
42