Lesson 3/Learning Event 1
SEMITRAILER AIR-BRAKE SYSTEMS
Most larger trailers equipped with airbrake systems use a service
air line and an emergency air line connection to the tractor that
tows the trailer. The trailer brake system consists of one or more
reservoirs, relay emergency valve, brake chamber and slack adjuster
for each wheel, and the connecting air lines.
When the emergency line is connected to the tractor and the
connection valve is opened, air will flow from the truck reservoir to
the trailer relay valve and into the reservoir. This raises the air
pressure at the trailer to the same pressure as that of the truck and
holds the quickrelease valve in the relay emergency valve open. The
compressed air in the trailer reservoir is now available to make the
trailer brakes work.
When the truck brake pedal is depressed or the trailer brake control
lever is moved, compressed air will move through the service line
between the tractor and trailer to the top of the relay emergency
valve on the trailer.
The compressed air will close the release valve and open another
valve in the relay emergency valve allowing air from the trailer
reservoirs to go to each wheel brake chamber on the trailer. This
applies the brakes.
When pressure on the service line is released, the air supply from
the trailer reservoir to the brake chambers is shut off. At the same
time, the quickrelease valve in the emergency relay valve is opened
so that the air in the brake chambers can escape and release the
The relay emergency valve is designed to lock the trailer brakes in
case the trailer ever breaks away from the tractor. If, for some
reason, the emergency air supply line becomes disconnected from the
tractor, there will be no air pressure applied where the air enters
the relay emergency valve. Without pressure the quickrelease valve
will close and allow air pressure from the reservoir to go to the
brake chambers. This will lock all brakes on the trailer. The
brakes will remain locked as long as the air pressure in the trailer
reservoir remains high enough. However, as soon as a supply of air
is hooked to the emergency line again, the pressure will open the
quickrelease valve, close off the air supply to the chambers, and
the brakes will release. The brakes can also be released by draining
the air from the reservoir.