Page 184 - Driving Commercial Vehicles Manual+

chapter 8 — air brakes

Trailer air supply valve

Once the supply line is connected to the trailer, there needs to be a way of
directing air pressure to the trailer.

Trailer air supply valve.

This is the job of the dash mounted trailer air supply valve. It senses air
pressure in the supply line that carries air to the trailer system. Most trailer air
supply valves are an octagon-shaped red button.

Hand valve
Applying the foot valve directs approximately the same application pressure
to both the tractor and trailer brakes. For example, if you make a 20 p.s.i. foot
valve application, this application pressure will be applied to both the tractor
and trailer brakes.
There are times when you want to apply only the trailer brakes without
applying the tractor brakes, such as when coupling the tractor to a trailer.

The hand valve allows
independent control of the
trailer brakes. Some are
mounted on the steering
column, others are mounted
in the dashboard.

This is the purpose of the hand valve. When the trailer air brake system is fully
warning! connected to the tractor, the hand valve allows you to apply the trailer brakes
independently of the tractor.
DO NOT use the hand valve
for parking. The hand valve should not be used in normal or emergency situation braking.
Always use the foot valve for service braking.
Apply the tractor parking
brakes, close the trailer Most hand valves are spring-loaded, just like the foot valve, so that when
supply valve and block the you release it, it will return to the released position. Don’t use the hand valve
trailer wheels. for parking.

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