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| Your proportioning valve is controlled by the air pressure in the trailer supply line. Thats how it knows wether there is a trailer attached or not, and wether it needs to proportion the brakes. I know of a few guys that have converted tractors to bobtail monure or silage trucks and wear out the front brakes within the first 20,000 niles because they ran with the red supply button out and the proportion valves though it was a bobtail. Push the red button in, now it gives full pressure to all axles, and all is right in the world agian.
Here is nice write up about proportioning valves from Bendix.
http://www.bendixvrc.com/itemDisplay.asp?documentID=2400
Edited by silagehauler 6/26/2011 00:27
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gooseneck to semi
