Trailer Brakes not staying in on a 15 Kenworth T800 Super 10

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Quickdumptrucking, Apr 15, 2024.

  1. Quickdumptrucking

    Quickdumptrucking Bobtail Member

    Jun 2, 2023
    Hello truckers and mechanics out there just wanted to share something that happened to me today with the hope that this info is useful to someone out there.
    This morning I took a trailer with equipment to a job site, after unloading the equipment, I was going to go park the trailer down the road but the red knob for the trailer air supply wouldn’t stay in, only the tractor air supply yellow knob would stay in and of course the trailer parking brakes would not release, I had good air pressure in both tanks and I tried pushing the knobs different ways with no success, after checking a few things I noticed air leaking out on the exhaust port of the dash control valve, so that lead me to conclude the dash valve must be the problem but I didn’t have a spare part on hand so I had no option but to drive the truck as it was of course leaving the trailer behind. After a few short necessary trips I began to notice a fast loss of air pressure every time I would hold the service brakes at a stop light, so once I arrived at the job site with another dash control valve on hand I asked a coworker to help me check the truck to find out where the air leak was coming from, as I pressed on the service brakes he found out that there was air coming out of both glad hands… after doing some research all pointed out to a faulty tractor protection valve as well but this bendix valve had only been lightly used for about 3 months so I doubted I would be so unfortunate as to have a faulty dash control valve and a faulty tractor protection valve so I began replacing the dash control valve, after I replaced it I filled up the air tanks and pressed the tractor supply yellow knob and to my surprise air was still leaking out of the glad hands when I pressed the service brakes, then I pressed on the trailer air supply red knob to check if it would stay in and it did with no air leaking out the exhaust port anymore so I decided to go connect to the trailer, I hooked up the red air supply hose, leaving the blue service brake hose disconnected to double check if the tractor protection valve was faulty, after pushing both knobs in, I pressed the service brakes and there was no more air leaking on the blue service glad hand, again I disconnected both glad hands, pushed only the tractor supply yellow knob and pressed the service brakes and no more air was coming out the glad hands, after that I hooked up the trailer and everything was working back to normal. So to summarize this long story, this faulty dash control valve caused my trailer air supply to fail and my tractor protection valve to act up.
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  3. Magoo1968

    Magoo1968 Road Train Member

    Mar 18, 2021
    St Malo mb
    Blue line should’ve been leaking air if red button was charged and you applied the brakes. I don’t think you fixed the problem it’s possible you have trailer brakes right now.. unless I’m reading it wrong I’d check with trailer spike to verify they work..
  4. SmallPackage

    SmallPackage Road Train Member

    Dec 20, 2019
    Marion Texas
    May have been a dirt duaber building in a glad hand. Held pressure back until the hoses were uncoupled and blew the blockage out of the system. Always a good idea to cover the glad hands when left parked and uncoupled. Keeps bugs and moisture out system.
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