Page 182 - Driving Commercial Vehicles Manual+

chapter 8 — air brakes

The system shown uses one reservoir and two air valves, a relay valve for the
driving tip service brakes, and a trailer spring brake valve that fills the reservoir and
controls the spring parking brakes.
This is how you can tell if you
have reversed the supply line Other systems may be equipped with one, two or three air valves and
and service line connections: multiple reservoirs. But using more or fewer air valves or additional reservoirs
• the spring parking brakes won’t alter the basic operation of the system.
won’t release even when The tractor is delivering air through the supply line to the trailer spring brake
you use the parking brake valve. The spring parking brake valve directs air to fill the reservoir(s) and to
control on the dashboard release the spring parking brakes.
• the trailer service brakes
won’t function. Here are the two types of systems:
Avoid these situations by • one fills the reservoir(s) before releasing the spring parking brakes
taking care in coupling to
a trailer. • the other releases the spring parking brakes first, then fills the reservoir(s).

Always perform a tug test after coupling the tractor to the trailer. Follow
the coupling procedures as shown in chapter 5, skills for driving trucks
driving tip and trailers.

Always perform a tug test Trailer with spring parking brakes — service brake application
after coupling the tractor to
a trailer. In this diagram, a control signal from the tractor has been sent through the
Before conducting a tug test, control line to the trailer’s relay valve. The relay valve has drawn air from
use a flashlight to visually the trailer reservoir and delivered it to the trailer service brake chambers at
check that the fifth wheel approximately the same pressure as the control signal.
locking jaws are closed
and locked.

The trailer brakes have been
applied by pressing on either
the foot valve or applying
the hand valve, or both.

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