Commercial Driver Handbook ( PDF ) - California Department of ...

trailer air tanks
Each trailer and converter dolly has one or more air
tanks. They are filled by the emergency supply line
from the tractor and they provide the air pressure
used to operate trailer brakes. Air pressure is sent
from the air tanks to the brakes by relay valves. The
pressure in the service line tells how much pressure
the relay valves should send to the trailer brakes.
The pressure in the service line is controlled by
the brake pedal and the trailer hand brake.
It is important that you do not let water or oil
build up in the air tanks. If you do, the brakes
may not work. Each tank has a drain valve on it,
and must be drained every day. If your tanks have
automatic drains, they will keep most moisture
out. However, you should still open the drains to
check for moisture.
shut-off valves
Shut-off valves (also called cut-out cocks) are used
in the service and supply air lines at the back of
the trailers used to tow other trailers. These valves
permit closing the air lines when no other trailer
is being towed. You must check that all shut-off
valves are in the open position except the ones at
the back of the last trailer, which must be closed.
serviCe, spring, and emergenCy
Brakes
Newer trailers have spring brakes just like trucks
and truck tractors. However, converter dollies and
trailers built before 1975 are not required to have
spring brakes. These have emergency brakes which
work from the air stored in the trailer air tank. The
emergency brakes come on whenever air pressure
in the emergency line is lost. These trailers have
no parking brake. The emergency brakes come
on whenever the air supply knob is pulled out or
the trailer is disconnected. The brakes will hold
only as long as there is air pressure in the trailer
air tank. Eventually, the air will leak away and
then there will be no brakes. It is very important
that you use wheel chocks when you park trailers
without spring brakes.
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A major leak in the emergency line will cause the
tractor protection valve to close and the trailer
emergency brakes to come on.
You may not notice a major leak in the service line
until you try to put the brakes on. Then, the air
loss from the leak will lower the air tank pressure
quickly. If it goes low enough, the trailer emergency
brakes will come on.
InSPectIng the AIR BRAke
SyStem
Use the basic inspection procedures described in
Sections 2 or 4 to inspect your vehicle. There are
more items to inspect on a vehicle with air brakes
than one without them.
engine Compartment
Check air compressor drive belt. If the air
compressor is belt-driven, check for excessive
wear, cracks, and tightness of the belt.
Walkaround
Check brake adjustment on S-cam brakes. Park
on level ground and chock the wheels to prevent the
vehicle from moving. Release the parking brakes
on the truck or tractor and the emergency brakes
on the trailer so you can mark the push rod in the
unapplied position. Make a mark on the push rod
with a chalk or scribe close to the brake chamber
where the push rod comes out of the air chamber.
Apply the truck or tractor parking brake and the
trailer emergency braking system. Measure the
travel of the push rod from the brake chamber
and the mark you made with the chalk or scribe
at each brake chamber. The push rod should move
less than one inch on most brakes. (Smaller brake
cams will have less push rod travel.)
If the brake push rod exceeds the required adjustment,
adjust it or have it adjusted. (You are not
expected to adjust them during the pre-trip test but
you are expected to describe how to check that the
brake push rod is adjusted properly.) Vehicles with
too much brake slack can be very hard to stop.
Out-of-adjustment brakes are the problem most
often found in roadside inspections. Be safe—check
the slack adjusters.