Trailer Control Valve
The independent trailer control valve (fig. 7-48)
provides the operator with control of the trailing load at
all times. This valve functions in the same manner as
the treadle valve except that the handle is turned, rather
than depressed, to operate the valve.
The quick-release valve (fig. 7-49) exhausts brake
chamber air pressure and speeds up brake release by
reducing the distance the air would have to travel back
to the brake valve exhaust port.
When the brakes are engaged, air from the brake
valve enters into the quick-release valve, forcing the
diaphragm down and closing off the exhaust port. This
action allows air pressure to rush through the
quick-release valve outlet ports to the wheel brake
chambers. When the brakes are released, the air
pressure above the quick-release diaphragm is
Figure 7-48.Trailer control valve.
Figure 7-49.Quick-release valve.
exhausted at the brake valve. As air pressure above the
diaphragm is released, the air pressure below the
diaphragm raises off the exhaust port. This action
allows the air in the brake chambers to exhaust at the
When air is leaking from the system, a leakage test
can determine if there is air leaking at the quick-release
valve. The leakage test is performed with the brakes
applied and coating the exhaust port with soapsuds. If
air bubbles form, this is a sign of a defective valve,
which can be corrected by either cleaning and
replacing worn parts or by replacing the unit. Dirt,
worn diaphragm, or a worn seat causes leakage.
Combined-Limiting and Quick-Release Valve
The combined-limiting and quick-release valve
(fig. 7-50) is used in combination with a two-way
check valve in the air brake system of trucks and
tractors. The combined-limiting and quick-release
valve is interchangeable in mounting with the
quick-release valve and serves the same purpose with
the additional function of providing an automatic
reduction of front-wheel brake pressure, at the option
of the operator, on slippery roads.
Tractor Protection Valve
The primary purpose of the tractor protection
valve (fig. 7-51) is to protect the tractor air brake
system under trailer breakaway conditions and under
conditions where severe leakage develops in the
tractor or trailer.
Figure 7-50.Combined-limiting and quick-release valve.