How to Test Trailer Air Brakes for Leaks


Air brake systems for tractor-trailer combinations have parts to control the trailer brakes. If the system leaks at any point, the brakes could fail. As part of your pre-trip inspection, check that air flows to all the trailers. Test the tractor protection valve, trailer emergency brakes and trailer service brakes.

  • Identify the air pressure gauge on the dashboard. This gauge shows how much air pressure there is in the air tank in pounds per square inch (psi). A dual air brake system has a gauge for each tank. Your vehicle may also have an application pressure gauge that shows how much air pressure you are applying to the brakes when you depress the brake pedal.

  • Start with the air brake system fully charged. The air pressure gauge should read about 125 psi. Turn off the engine. Exit the tractor. Chock the wheels. Return to the tractor and release the brakes by pulling out the parking brake valve, a yellow, diamond-shaped knob. Let the vehicle stand for one minute.

  • Observe the drop in air pressure. The gauge should show a drop of no more than three pounds psi leakage in one minute.

  • With the treadle valve or foot brake, apply 90 psi of pressure and wait for one minute. Note the reading on the air pressure gauge. For a tractor-trailer combination, the drop in air pressure should be no more than 4 psi per minute.

  • With the treadle valve or foot brake, pump down the air pressure until the warning buzzer comes on. This should occur before the air pressure reaches 60 psi. If it doesn't, the low air pressure warning device is defective.

  • Again, with the treadle valve or foot brake, pump the brakes until spring brakes pop out. They should apply at between 20 psi and 40 psi. If they apply above 45 psi, the system is defective.

  • Turn the ignition key and start the engine. Allow the air pressure to build. The air pressure should reach 85 to 100 psi in 45 seconds at operating RPM.

  • Exit the tractor. Remove the chocks and get back in the tractor. Push in the clutch and, using the shift lever, shift the transmission into first gear. Release the trailer brakes and ease up on the clutch to test the parking brake.

  • To test the parking brake, start with the parking brake applied. Release the parking brake as described in Step 1 and let the truck roll forward at about 5 mph. If the parking brakes work properly, the vehicle should not move.

  • With normal air pressure and the parking brake released, move the vehicle forward slowly. Push in the clutch and apply the foot brake to see if there is any unusual pulling to the left or to the right.

Tips & Warnings

  • Listen for any air leaks, especially behind the dashboard and under the cab and trailer.
  • Department of Motor Vehicle practices vary from state to state, but applicants for a Commercial Driver License without an Air Brake Restriction should be prepared to describe or demonstrate all these steps from memory.
  • Don't forget to turn the tractor off once air pressure is built.

Related Searches


  • "Bumper to Bumper, The Complete Guide to Tractor-Trailer...";Mike Byrnes & Assoc., Inc.; 2010
  • "Barron's Commercial Driver's License Truck Driver's Test"; Mike Byrnes & Assoc., Inc.; 2010
  • "Practical Airbrakes"; Rolf VanderZwaag; 2006
  • Photo Credit trucks on highway image by palms from
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