Trailer Air Lines
Every combination vehicle has two air lines. These
lines are the service line and the emergency line. They
run between each vehicle, such as tractor to trailer,
trailer to dolly, and dolly to second trailer.
SERVICE AIR LINES. The service line carries
air that is controlled by the foot brake or the trailer hand
brake. Depending on how hard the foot brake is
engaged, the pressure in the service line will similarly
change. The service line is connected to relay valve(s)
on the trailer to apply more or less pressure to the trailer
brakes. As pressure increases in the service line, the
relay valve opens and sends air pressure from the trailer
air tanks to the trailer brake chambers, thus applying the
EMERGENCY AIR LINES. The emergency
line has two purposes. First, it supplies air to the trailer
air tanks. Second, the emergency line controls the
emergency brakes on the combination vehicle. Loss of
air pressure in the emergency line causes the trailer
emergency brakes to activate. The pressure loss could
be caused by a trailer breaking loose and tearing apart
the emergency air hose. The loss could also be the result
of a hose, metal tubing, or other parts breaking and
causing an air leak. When the emergency line loses
pressure, it also causes the tractor protection valve to
close, causing the air supply knob to pop out.
Emergency lines such as hoses couplers, and other
parts, have a red covering. The red covering allows you
to separate the emergency lines from the service lines
which have a blue covering.
Hose couplers, commonly known as glad hands
(fig. 7-11), are coupling devices used to connect the
service and emergency air lines from the truck or tractor
to the trailer. The glad hands have rubber seals, known
as rubber grommets, that prevent the air from escaping.
Clean the rubber grommets before you connect the glad
hands. When connecting the glad hands, press the two
seals together with the glad hands at a 90-degree angle
to each other. A turn of the glad hands attached to the
hose joins and locks the couplers.
Some vehicles have dead end or dummy glad
hands to which the hoses should be connected when not
in use. This prevents water and dirt from getting into the
glad hands and the air lines. This is very important
because keeping the air system clean is a critical factor.
Figure 7-11.Glad hands.
When connecting the glad hands, ensure the proper
glad hands are coupled together. On some equipment,
metal tags are attached to the lines with the words
service and emergency stamped on them. The color
blue is used for the service line and the color red for the
emergency line connections.
If the air lines are crossed, supply air is sent to the
service line instead of going to charge the trailer air
tanks; therefore, air is not available to release the trailer
spring brakes (parking brakes). If the spring brakes do
not release when you push the trailer air supply control
knob, check the air line connections.
Older trailers do not have spring brakes. If
the air supply in the trailer air tanks has leaked
out, emergency brakes will not exist, and the
trailer wheels will turn freely. If you cross the
air lines, the trailer will roll; however, there will
be no trailer brakes.
NOTE: Always test the trailer brakes before driving
by engaging the hand valve or by pulling the tractor
protection valve. Once these brakes are engaged, shift
the tractor to low gear and pull gently against the brake
system to make sure the brakes work.
Shutoff valves, commonly known as cutoff cocks,
are used in the service and emergency lines glad hands
located on the back of military series tractors, cargo