Air Brake Manual

Hand Valve Application
30 26
31
In this illustration with the foot valve (31) released and
the hand valve (30) opened, application air is directed
from the hand valve through a two-way check valve
(26), to the brake chambers. The two-way check valve
in this application has shifted to the low pressure side,
closing off any air flow toward the foot valve side.
Any time a trailer brake application is made using the
hand valve, the driver may depress the foot valve
treadle. If the foot valve application is of a higher
pressure than that of the hand valve, the two-way
check valve will shift to the lower pressure side,
allowing the higher pressure to be directed to the
tractor and trailer brakes. During a foot valve
application, if the driver makes a hand valve
application of a higher air pressure, the two-way
check valve will direct the higher hand valve air
pressure to the trailer brakes.
Although the trailer brakes may be applied independently
by means of the hand valve, the maximum
application pressure can only be the same as, or
slightly less than, reservoir pressure.
Tractor Protection System
A tractor protection system prevents total loss of air
from the tractor if the trailer breaks away, or if the
connecting air lines between tractor and trailer
become separated or ruptured. The tractor
protection system consists of two valves: the tractor
protection valve and the trailer supply valve. Other
names for the trailer supply valve are “trailer
parking control” and
“emergency valve.”
There are two types of trailer supply valves. The
most common is the automatic trailer supply valve.
This is a spring-loaded valve that is opened
manually and held open by air system pressure. The
other is a manual trailer supply valve, which may be
a toggle-type switch or a push/pull-type valve.
To understand the function of the trailer supply valve
and the tractor protection valve in the system, it is
important to understand how they operate.
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