Page 54 - 2021-2022 Commercial Drivers Manual

SECTION 6 : COMBINATON VEHICLES

TEST YOUR KNOWLEDGE 6.2.5 – Hose Couplers (Glad Hands)

Glad hands are coupling devices used to con-
Subsection 6.1 nect the service and emergency air lines from
the truck or tractor to the trailer. The couplers
• What two things are important to prevent • What is offtracking? have a rubber seal, which prevents air from
rollover?
• When you back a trailer, you should escaping. Clean the couplers and rubber seals
• When you turn suddenly while pulling position your vehicle so you can back in before a connection is made. When connecting
doubles, which trailer is most likely to a curved path to the driver’s side. True or the glad hands, press the two seals together
turn over? False? with the couplers at a 90 degree angle to each
other. A turn of the glad hand attached to the
• Why should you not use the trailer hand • What type of trailers can get stuck on hose will join and lock the couplers.
brake to straighten out a jackknifing railroad-highway crossings? When coupling, make sure to couple the
trailer? proper glad hands together. To help avoid mis-
These questions may be on your test. If you can’t answer them all, re-read subsection 6.1. takes, colors are sometimes used. Blue is used
for the service lines and red for the emergency
(supply) lines. Sometimes, metal tags are at-
tached to the lines with the words “service” and
“emergency” stamped on them. See Figure 6.6
If you do cross the air lines, supply air will
be sent to the service line instead of going to
charge the trailer air tanks. Air will not be avail-
Correct Drift Immediately. As soon as you 6.2.3 – Trailer Air Supply Control able to release the trailer spring brakes (parking

see the trailer getting off the proper path, cor- brakes). If the spring brakes don’t release when
rect it by turning the top of the steering wheel The trailer air supply control on newer ve- you push the trailer air supply control, check
in the direction of the drift. hicles is a red eight-sided knob, which you use to the air line connections.
control the tractor protection valve. You push it Older trailers do not have spring brakes. If
Pull Forward. When backing a trailer, make in to supply the trailer with air, and pull it out to the air supply in the trailer air tank has leaked

pull-ups to re-position your vehicle as needed. shut the air off and put on the trailer emergency away there will be no emergency brakes, and the

brakes. The valve will pop out (thus closing the trailer wheels will turn freely. If you crossed the

6.2 – Combination Vehicle tractor protection valve) when the air pressure air lines, you could drive away but you wouldn’t
Air Brakes drops into the range of 20 to 45 psi. Tractor pro- have trailer brakes. This would be very danger-
tection valve controls or “emergency” valves on ous. Always test the trailer brakes before driv-
older vehicles may not operate automatically.
You should study Section 5: Air Brakes before ing with the hand valve or by pulling the air
reading this. In combination vehicles the brak- There may be a lever rather than a knob. The supply (tractor protection valve) control. Pull
ing system has parts to control the trailer brakes, “normal” position is used for pulling a trailer. gently against them in a low gear to make sure
in addition to the parts described in Section 5. The “emergency” position is used to shut the the brakes work.
These parts are described below. air off and put on the trailer emergency brakes. Some vehicles have “dead end” or dummy
couplers to which the hoses may be attached
6.2.1 – Trailer Hand Valve 6.2.4 – Trailer Air Lines when they are not in use. This will prevent wa-

Every combination vehicle has two air lines, ter and dirt from getting into the coupler and
The trailer hand valve (also called the trolley the service line and the emergency line. They the air lines. Use the dummy couplers when

valve or Johnson bar) works the trailer brakes. run between each vehicle (tractor to trailer, the air lines are not connected to a trailer. If

The trailer hand valve should be used only to trailer to dolly, dolly to second trailer, etc.) there are no dummy couplers, the glad hands
test the trailer brakes. Do not use it in driving can sometimes be locked together (depending

because of the danger of making the trailer skid. Service Air Line. The service line (also called on the couplings). It is very important to keep

The foot brake sends air to all of the brakes on the control line or signal line) carries air, which the air supply clean.
the vehicle (including the trailer[s]). There is is controlled by the foot brake or the trailer hand

much less danger of causing a skid or jackknife brake. Depending on how hard you press the

when using just the foot brake. foot brake or hand valve, the pressure in the
Never use the hand valve for parking be- service line will similarly change. The service
cause all the air might leak out unlocking the line is connected to relay valves. These valves al-

brakes (in trailers that don’t have spring brakes). low the trailer brakes to be applied more quickly

Always use the parking brakes when parking. than would otherwise be possible.
If the trailer does not have spring brakes, use Emergency Air Line. The emergency line (also
wheel chocks to keep the trailer from moving.
called the supply line) has two purposes. First, it
supplies air to the trailer air tanks. Second, the
6.2.2 – Tractor Protection Valve
emergency line controls the emergency brakes
The tractor protection valve keeps air in the on combination vehicles. Loss of air pressure in

tractor or truck brake system should the trailer the emergency line causes the trailer emergency

break away or develop a bad leak. The tractor brakes to come on. The pressure loss could be
protection valve is controlled by the “trailer air caused by a trailer breaking loose, thus tearing

supply” control valve in the cab. The control apart the emergency air hose. Or it could be

valve allows you to open and shut the tractor caused by a hose, metal tubing, or other part
protection valve. The tractor protection valve breaking, letting the air out. When the emer-

will close automatically if air pressure is low gency line loses pressure, it also causes the

(in the range of 20 to 45 psi). When the tractor tractor protection valve to close (the air supply

protection valve closes, it stops any air from go- knob will pop out).
ing out of the tractor. It also lets the air out of the Emergency lines are often coded with the
trailer emergency line. This causes the trailer color red (red hose, red couplers, or other parts)

emergency brakes to come on, with possible loss to keep from getting them mixed up with the

of control. (Emergency brakes are covered later.) blue service line.
Figure 6.6
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