When i connect a semi trailer to my truck and i release park brake a red warning light comes up on the dash saying trailer air,this light turns off as soon as I apply the park brake the trailer hasn't got any air leaks any ideas
By releasing the park brake you are in fact putting air in the park brake (spring brake diaphragms) that compresses the springs and allows the brakes to release. When you apply the park brake you are in effect dumping the air out of the system and allowing the spring brakes to come on. The light is telling you that there is insufficient air in the park brake circuit to fully release the spring brakes. Some ideas are that the air lines on the trailer are opposite to the prime mover(tractor), there is a kink in the air lines in the trailer , the air tank that holds emergency air for the spring brakes system has an internal leak in a valve between chambers or externally between air tanks. I would make very sure that I fixed this problem before I travelled as the consequences of a trailer wheel lock up at speed could have very serious results.
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