Air Brake Manual
Air Brake Manual
Air Brake Manual
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Loss of Reservoir <strong>Air</strong> Pressure<br />
28<br />
21<br />
In this illustration the pressure has been lowered to<br />
approximately 45-20 psi and the tractor protection<br />
system has closed automatically, placing the trailer<br />
brakes into an emergency position. Also, the spring<br />
parking brakes system has had the air pressure<br />
released activating the spring parking brakes.<br />
The truck protection system described is an<br />
example of a tractor equipped with a type of cabmounted<br />
trailer supply valve (28) which will close<br />
automatically when the air pressure in the supply<br />
(emergency) line (21) drops below 45-20 psi. The<br />
valve may also be closed manually.<br />
<strong>Manual</strong> Trailer Supply Valve<br />
Some older tractors may be equipped with a different<br />
type of cab-mounted trailer supply valve which must<br />
be operated manually by the driver. It has two<br />
positions: normal and emergency.<br />
The tractor will be equipped with a tractor protection<br />
valve, and the trailer unit with a relay emergency<br />
valve, as in the previous system.<br />
The functions of the trailer supply valve, tractor<br />
protection valve and the relay emergency valve will<br />
be similar to those explained previously. However,<br />
there is one important difference. In the event of<br />
tractor reservoir air loss, the trailer supply valve must<br />
be shifted to the emergency position manually to seal<br />
off the tractor.<br />
Any time the driver shifts the cab-mounted trailer<br />
supply valve to the emergency position, and the<br />
trailer system is charged, the trailer supply valve will<br />
exhaust the supply (emergency) line, which causes<br />
the trailer reservoir to dump its air directly to the<br />
trailer brake chambers.<br />
The trailer brakes will remain applied only as long as<br />
air pressure remains within the trailer system. How<br />
long the air in the system will hold the brakes applied<br />
depends on how airtight the system is. As a safety<br />
precaution, parked trailers without spring parking<br />
brakes should always have the wheels blocked to<br />
prevent a possible runaway. To move a trailer that has<br />
been parked with the brakes in an emergency<br />
application, it is necessary to charge the system to<br />
release the trailer brakes.<br />
51
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