In a tractor-semitrailer, the lines that provide the compressed air to the air brake system run through the entire vehicle combination. A leak in the trailer part of the air system could prevent the tractor brakes from working. The tractor protection valve keeps air in the tractor or truck brake system even if the semitrailer breaks away or develops a bad leak. That way, the tractor's brakes will continue to work. The tractor protection valve itself is located at the point under the tractor's frame where the flexible airlines that go to the trailer are connected by the coupling devices called glad hands.
Connect the service and emergency brakes of your trailer to the tractor air supply system using the glad hands. These hose coupling devices on the ends of the air lines on the back of your tractor and on the front of your trailer are often color-coded so that you can connect them properly. The service brake air line couplers are colored blue and the emergency brake air line couplers are colored red. The coupling device is a snap-lock type, similar to a radiator cap. Press the seals of the service brake air line couplers together, with the couplers at a 90 degree angle to each other. Rotate the glad hand attached to the hose to join and lock the couplers. Couple the glad hands of the emergency brake line the same way.
Test the tractor protection valve with air pressure at about 90 lbs. and the engine off. Push and release the foot brake until the low air pressure warning comes on, which should happen when the air pressure reaches 60 psi. On the dashboard, a red, eight-sided knob (often marked "Trailer Air") that controls the tractor protection valve should pop out after the low air warning device activates. The spring brakes should apply automatically between 20 and 40 psi.
To test the tractor protection valve using a different method, charge the trailer air lines by pushing in the red tractor protection valve control knob. Disconnect the trailer supply/emergency line by uncoupling the glad hands for that line. Air escapes and the tractor protection valve should pop out between the air compressor cut-out point (120 psi to 135 psi) and 20 psi.
The low air pressure warning device activates if the trailer system loses air pressure below around 20 to 45 psi, and a spring in the trailer air supply valve on the dashboard automatically pops the red knob out. This action sends a signal to the tractor protection valve to close off the air supply to the trailer, protecting the tractor air supply from loss so that those brakes will continue to work, and activating the trailer emergency brakes. Operate the tractor protection valve manually by pulling out the trailer air supply control (red knob). This shuts off the air supply to the trailer and applies the trailer emergency brakes. To resupply the trailer brakes with air, push the red knob in.
Tips & Warnings
- If you hear air escaping from the glad hands, uncouple them. Examine the seals. They should be undamaged and free of any dirt or sand that could cause leaks.
- Keep replacement rubber glad hand seals on hand. They can become damaged over time but are fairly simple to replace. Having replacements on hand minimizes downtime.
- Spring brakes that activate when air pressure is above 45 psi should alert you to a defect in the system.
- "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
- "Bumper to Bumper Easy CDL"; Mike Byrnes & Assoc., Inc.; 2010
- "Practical Airbrakes"; Rolf VanderZwaag; 2006
- Photo Credit brake cables image by max blain from Fotolia.com
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