Help tracking down air leak

Discussion in 'Trucks [ Eighteen Wheelers ]' started by Hanadarko, Aug 13, 2010.

  1. Hanadarko

    Hanadarko Independent Owner/Operator

    Oct 1, 2009
    I started a new thread for this new problem. I was 100% sure I nailed down the air leak I heard. I had tested the Bendix Trailer/Tractor valve and was sure that it was leaking out the yellow valve stick piece. I replaced it and the same #### thing is happening. I even used all new airline fittings and made sure my hoses were cut perfectly clean and true.

    What I am seeing is this:

    120psi in the system and I apply (push in) the yellow valve. I hear the tractor brakes release and then I hear a small air flow inside the dash...this keeps going on as long as the yellow valve is pushed in. I pop out the yellow valve and it goes quiet.

    All of the fittings on the dash valve are TIGHT and not leaking.

    I then pinched off both of the tractor parking brake supply lines and then pushed in the yellow tractor valve. I still hear the air hiss and the parking brakes stayed applied. So I dont think I have a leaking brake chamber here. I also sprayed all of the connections in the back of the truck from the tractor protection valve on the back of the sleeper to the brake chambers. Nothing is leaking anywhere.

    How do I continue to troubleshoot this and what else might the yellow valve do (when pushed in) that I could check over??

    :biggrin_25513: - I am really loosing it here. Literally.

    Now...maybe 2 out of 5 times I push in the YELLOW valve, it releases the parking brakes and goes totally quiet. That is the odd part.
    At least when the truck is running, the system air can more than cover this leak but when it is leaking, I am loosing about 20psi in under 20mins.

    HELP!! :biggrin_25526:
  2. rocknroll nik

    rocknroll nik High Risk Load Member

    Oct 18, 2008
    can't read the sign
    I had the same problem with my freight isn't always at the nob, I know this sounds stupid but did you check the glad hands and seals, the glad hands on my truck were not in the best shape so they didn't always make a good seal, I kept asking for new ones but was told that there was nothing wrong with that truck. It seemed that when I made sure there was a very tight seal at the hands I didn't get that air hissing .... I don't know what else to tell ya driver...good luck
  3. lilillill

    lilillill Sarcasm... it's not just for breakfast

    Nov 7, 2007
    Is there air coming out of the exhaust port on the relay valve with the yellow knob pushed in?

    I had that on a trailer that was dug out of the weeds... after being out of commission for a while, the relay valve piston was partially froze up. After mucho WD-40 down the red air line, it finally freed itself up.

    I'm pretty sure there is a similar valve on the tractor. How long was this truck sitting before you got it?
  4. melpromud

    melpromud Medium Load Member

    Jan 29, 2010
    camden ny
    Their was just a tread on this less than a week ago on hear. Just because you hear air moving at the dash dont mean that is where the leak is. All you are hearing is air moving through the valve to the leak else where. Check your break chambers! That seams to be where the leak shows up most of the time. If its not the break chambers it can be any where in the primary side air system.
    Build full air pressure, push in the break valve and turn off the truck. Get out and listen. The leak might be small but you should still be able to hear it.
    tesla4all Thanks this.
  5. RenegadeTrucker

    RenegadeTrucker Road Train Member

    Dec 25, 2009
    Get a glad hand and put a pipe cap in it then put it on your truck, hit the handle and see if you still get the leak. Doing so will tell you if the leak is in the tractor or the trailer. Also, glad hands are natorious for being intermitent leakers. And they do not have to have cracks on them, if one got pulled a little too hard while making a corner or backing into a dock the housing can become slightly bent and will not always seal and can create an intermitent leak depending on how much tension is on the air line it is attached to, and depending on what direction the tension is from.

    This is why I always ran quick connects like you have for the air tools in your shop, just much larger ones. If I had to hook to some one else'es trailer I had a set of airlines with the quick connects on one end and gladhands on the other.

    It also meant my trailer would have been a lot harder to steal because it had no glad hands on it, and air lines were under the step by the landing gear, not on the front of the trailer, I liked that setup much better.

    You might consider going to it as well because it allows you to keep a spare set of airlines incase yours get tore up, there is nothing worse than being shut down 1am in the middle of nowhere simply because one of your pig tails split on you. With this setup you can change them out in less than 30 seconds and it requires no tools.
    rocknroll nik Thanks this.
  6. Hardlyevr

    Hardlyevr Road Train Member

    Jul 30, 2009
    umm, if he is hearing it with the yellow valve, then it doesn't have anything to do with the gladhands or the trailer! The yellow valve controls air to the tractor brake system. I had one like this that replacing fixed the leak. My curent tractor does this on occasion, and wiggling it a bit makes it quit.
  7. end of the road

    end of the road Heavy Load Member

    Jul 4, 2010
    Northern Canada
    If you wiggle it and it quit, you need to replace the valve....again.

    If it doesn't, get the soapy water and get busy.
  8. Hanadarko

    Hanadarko Independent Owner/Operator

    Oct 1, 2009
    Yea, I am bobtail at the moment (only). I have isolated the brake chambers - they are not the problem.
    The glad hands/hoses are all new - but not leaking at all. This is only a tractor issue as you said.
    I have tried to wiggle it and stuff and no help...wiggling it makes no difference this time - Thats why I replaced it but the new (I mean BRAND NEW) unit still does the exact same thing.

    I am going to soap up the crap out of the valve and all of its fittings again Saturday...Last time a fitting was bad, it was the brown hose (Supply from main tank)....

    Dam stupid thing... Now, it was sitting intermittently for 1 yr but driven every 2 weeks. Where might a relay valve be on the tractor side?
    I just went back outside, now thats its quieter out. I hear air surging inside the dashboard but NOWHERE outside of the rig. The only place I haven't checked yet is the main tank and secondary tanks themselves.
  9. james3505

    james3505 Light Load Member

    Jan 21, 2010
    try letting the truck air up completely, the push in the button and then stomp on the brake pedal hard and hold it for about 3 seconds.

    i had a truck that used to do this same thing and we replaced many parts and it never fixed it but the brake trick worked every time.
  10. bender

    bender Road Train Member

    Jul 20, 2010
    Don't Kid Yourself
    When you check your parking brake chambers for leaking internally to the service side, clamp the service lines at the chambers. Push yellow knob in. If a chamber is leaking internally you will see that chamber pushrod extending as the leak builds pressure. It will be applying the brake on its own.
  • Draft saved Draft deleted