Page 2 - Combination Vehicles Test Study Guide for the CDL

Combination Vehicle Air Brakes

The material in this section assumes that you have already read our study guide for the Air Brakes Test. Do so before reading this section—or you will be thoroughly confused.

Summary of the Information Found in the Study Guide “Air Brakes Test”

The Study Guide “Air Brakes Test” describes the operation of:

  • trailer protection valve
  • trailer air supply control (*Hint: This might be on your written DMV test.)
  • trailer air tanks
  • trailer service, parking, and emergency brakes
  • anti-lock braking system (ABS)

Review the material in our “Air Brakes Test” study guide before taking the written DMV test for combination vehicles.


  • The trailer protection valve cuts off the air supply to the trailer—in case the trailer breaks free or there is a massive air leak in the trailer.
  • The trailer air supply control is the red, eight-sided control knob on the dashboard of the truck—which controls the trailer protection valve and will “pop out” if the air pressure gets too low.
  • The Trailer Air Tanks store pressurized air in the rear of the trailer, near where it will be needed. Air from the tanks (controlled by the relay) is used to activate the trailer brakes.
  • The trailer service, parking, and emergency brakes are the three basic brake systems on modern tractors and trailers.
  • The ABS system prevents wheels from locking up and improves control of the vehicle during hard braking. ABS does not decrease stopping distance.

You should always use chocks when parking a trailer that does not have spring brakes. If air leaked from the trailer, the trailer could roll away.

If your trailer is equipped with ABS, there will be an ABS indicator light on the left rear corner of the trailer (rarely, it is found on the left nose of the trailer). This indicator will be yellow and will be labeled “ABS.”

The ABS indicator light can be seen in the driver-side mirror and will flash several times when the system is started. This indicator light will then switch off (if the ABS system is running normally). If the ABS indicator light continues to flash (or remains constantly “on”) there is a problem with your ABS system; contact a mechanic.

Trailer Hand Valve

The trailer hand valve (“Trolly Valve” or “Johnson Bar”) operates only the trailer brakes. It should only be used to test the trailer brakes. Do not use the trailer hand valve while driving. This could cause the trailer brakes to lock up, and then cause the trailer to skid or jackknife.

The foot brake operates the trailer and the tractor brakes, greatly reducing the risk of skid or jackknife.

Never use the trailer hand valve for parking. If the air leaked out of the system, the brakes could release—allowing the trailer to roll. When parking, use the parking brake. If the trailer does not have spring brakes—also use chocks.

Trailer Air Lines

There are two trailer air lines:

  • service air line “blue” (also called the “control line” or “signal line”)
  • emergency air line “red” (also called the “supply line”)

The service air line (which is color coded blue) provides the signal to the relay valves (located near the brake cylinders). The relay valves use pressurized air from the trailer air tanks to operate the brakes on the trailer. The service air line is controlled by both the foot brake and the trailer hand valve. The harder you press the foot pedal (or pull the hand valve), the more braking force is applied.

The emergency air line (which is color coded red) provides air for two uses. First, the emergency air line provides air to the trailer’s air tanks. Air from the trailer air tanks operate the trailer brakes (controlled by the relays). Second, the emergency air line controls the emergency brakes. Loss of air pressure in the emergency line causes the trailer emergency brakes to slam on.

Note: Air from the trailer air tanks is also used by the suspension system to fill the bellows for air ride. This has nothing to do with the brakes.

Hose Couplers (Glad Hands)

Glad hands are pneumatic couplers. They connect the air system of the tractor to the air system of the trailer. They each have a rubber seal (gasket). Before use, be certain that all four seals are clean and free of cracks. Attach service to service. Attach emergency to emergency. The glad hands will be color coded and/or marked “Service” and “Emergency.”

If you cross the lines, the trailer air tanks will not charge and the brakes will not release.

When not in use, the tractor’s glad hands should be inserted into the dummy coupler (a glad-hand holder bolted to the rear of the tractor). This keeps dirt and moisture out of the lines. If your tractor does not have a dummy coupler, link the two tractor glad hands together, and hook them to the back of the tractor, out of the way. Keep dirt and water out of the air system.

Always test the brakes on your trailers before driving. Set the trailer brakes and lightly pull against the trailer in low gear.

Note: Always have spare glad hand gaskets with you. It is common to find cracked or missing glad hand seals on trailers.


Retrieved from:

Shut-Off Valves

Shut-off valves are used at the rear of trailers that are used to pull other trailers (and on the rear of converter dollies). Each trailer (and dolly) has two shut-off valves—one for “service” and one for “emergency.” All shut-off valves must be open—except the two shut-off valves on the very rear of the truck, which must be closed.

Test the shut-off valves:

  • Set tractor brakes or chock wheels.
  • Charge the emergency line. (Press the trailer air supply control—red knob.)
  • Supply air to the service line. (Use the trailer handbrake lever.)
  • Go the rear of the rear trailer.
  • Open the shut-off valve on the emergency line. You should hear air escaping.
  • Close the shut-off valve.
  • Open the shut-off valve on the service line. You should hear air escaping.
  • Close the shut-off valve.
  • Set parking brake.

Note: Shut-off valves are also known as cut-out cocks. Cocks are a type of valve that turn 90 degrees from full open to full closed. Unlike regular valves that can be open, closed, or partially open/closed—cocks are either 100% open or 100% closed.

Hint: The term cut-out cock might be on your written DMV test.

All Study Guides for the CDL are now available as downloadable PDFs