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7.4 Doubles/Triples Air Brake Check

Perform the following checks in addition to Section 5.3: Inspecting Air Brake Systems

Check the brakes on a double or triple trailer as you would any combination vehicle. Section 6.2 explains how to check air brakes on combination vehicles. You also must make the following checks on your double or triple trailers:

Check air flow to all trailers:

Use the tractor parking brake and or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly (5) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

Test tractor protection valve:

Charge the trailer air brake system, (i.e., build up normal air pressure and push the “air supply” knob in). Shut the engine off. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from “normal” to “emergency” position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

Test trailer emergency brakes:

If the tractor protection valve does not work correctly, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.

Test trailer service brakes:

Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called tractor protection valve control or trailer emergency valve) or place it in the “emergency” position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.

Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve, but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)

Test Your Knowledge

  • What is a converter dolly?
  • Do converter dollies have spring brakes?
  • What three methods can you use to secure a second trailer before coupling?
  • How do you check to make sure trailer height is correct before coupling?
  • What do you check when making a visual check of coupling?
  • Why should you pull a dolly out from under a trailer before you disconnect it from the trailer in front?
  • What should you check for when inspecting the converter dolly? The pintle hook
  • Should the shut-off valves on the rear of the last trailer be open or closed? On the first trailer in a set of doubles? On the middle trailer of a set of triples?
  • How can you test that air flows to all trailers?

Study sections 7.1 through 7.4 if you can't answer all of these questions.

Many trucks do not come with trailer hand brakes. If your truck does not have one, simply check the emergency line on the rear trailer cut off valve.
The vast majority of trucks are designed so that the tractor protection valve will close (pop out) when air pressure is between 20 and 45 psi. This should be memorized.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

Doubles:

Refers to pulling two trailers at the same time, otherwise known as "pups" or "pup trailers" because they're only about 28 feet long. However there are some states that allow doubles that are each 48 feet in length.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

The shut-off valves on the middle trailer when pulling triple trailers should be in which position?
  • Both the service line and emergency line should be in the open position
  • The service line should be open but the emergency line should be closed
  • The service line should be closed but the emergency line should be open
  • Both the service line and emergency line should be in the closed position

Quote From The CDL Manual:

Check air flow to all trailers: Use the tractor parking brake and or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly (5) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

TruckingTruth's Advice:

Unless it is the very last trailer, all shut-off valves should remain in the open position so that all trailers have proper airflow in the air brake system.

Next
When pulling two or more trailers, the shut off valves on the first trailer should be in the following positions:
  • Both the service line and the emergency line should be open
  • The service line should be open and the emergency line should be closed
  • Both the service line and the emergency line should be closed
  • The service line should be closed and the emergency line should be open

Quote From The CDL Manual:

Check air flow to all trailers: Use the tractor parking brake and or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly (5) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

TruckingTruth's Advice:

Unless it is the very last trailer, all shut-off valves should remain in the open position so that all trailers have proper airflow in the air brake system.

Prev
Next
Should the shut-off valves on the rear of the last trailer be open or closed?
  • The service line should be open and the emergency line should be closed
  • The service line should be closed and the emergency line should be open
  • Both the service line and emergency line should be in the closed position
  • Both the service line and emergency line should be in the open position

Quote From The CDL Manual:

Check air flow to all trailers: Use the tractor parking brake and or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly (5) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

TruckingTruth's Advice:

All shut-off valves on the rear most trailer should always be in the closed position.

Prev
Next
What is a proper way to check the Tractor Protection Valve when pulling double or triple trailers?
  • On double and triple trailers, the tractor protection valve will not work
  • Step on and off the brake pedal several times to reduce the air pressure in the tanks until the trailer air supply control pops out
  • Chock the wheels, release all brakes, then go to the last trailer and open the shut off valve to release as much air from the system as possible to see if the trailer air supply control pops out
  • Engage and disengage the parking brake repeatedly until the trailer air supply control pops out

Quote From The CDL Manual:

Test tractor protection valve: Charge the trailer air brake system, (i.e., build up normal air pressure and push the "air supply" knob in). Shut the engine off. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

Prev
Next
Which of the following is NOT one of the steps involved in checking airflow to all trailers?
  • If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly are in the OPEN position
  • Use the tractor parking brake and or chock the wheels to hold the vehicle
  • Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged
  • Wait for air pressure to reach normal, then push in the yellow "tractor air supply" knob

Quote From The CDL Manual:

Check air flow to all trailers: Use the tractor parking brake and or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly (5) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

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