Losing air pressure when braking hard or coming to a stop.

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by spindrift, Oct 27, 2021.

  1. Rideandrepair

    Rideandrepair Road Train Member

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    What kind of shop can’t find an air leak? Small leak OK, big leak? WTF? Don’t make Me come down to Texas Boy!
     
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  3. spindrift

    spindrift Road Train Member

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    LOL
     
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  4. uncleal13

    uncleal13 Road Train Member

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    I have a Robinaire Ultrasonic leak detector. Best $600 I ever spent. saves so much time.
     
  5. Dino soar

    Dino soar Road Train Member

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    Your first post said you are losing pressure bobtail as well as being connected to a trailer, is that correct?

    I noticed that you said the leak dropped down to 50 lb. Did it stay at 50 lb?

    The reason I ask is because there are valves on your truck for auxiliary equipment- air seat, air bags, air horn, air operated heater/ defroster controls, things like that.

    Those valves will only allow like say your air seat or your airbags to fill if there's at least around 60 lbs. If your leak comes down to that point and it doesn't leak any farther then you probably are chasing something in the secondary Supply. It is set up that way so that if you have a leak in that secondary Supply that they all shut down and leaves that air for your brakes.

    If you think this is accurate:

    Are you sure it isn't something under your dash or any air controls underneath there if there are any?

    Are you sure it isn't something under the hood like maybe your Air Supply to your fan solenoid?

    Airbag or leveling valve connections?

    If not consider getting the ultrasonic leak detector. I need to order it I think it's only about $50 or so on Amazon.
     
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  6. spindrift

    spindrift Road Train Member

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    Yes, leaking air when bobtail AND hooked. I'll need to troubleshoot further in order to determine whether or not pressure drops below 50psi.
     
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  7. SmallPackage

    SmallPackage Road Train Member

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    May not be a leak at all. May just be the system passing more air then normal.
    When working the peddle or pulling the trailer hand valve does the exhaust of air sound like it is louder and has more volume of air escaping then was normal? (Higher pitched sqeel or longer, stronger purge sound when releasing foot pressure on peddle) If so i’d look at the foot valve. It may be letting more air then it should thru the exhaust port.
    Also the relay valve could also be sticking the same way and allowing more air to exhaust then normal. They will not make a sound or show a leak until the pedldle energizes them.
     
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  8. uncleal13

    uncleal13 Road Train Member

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    Check for water in air tanks.
    Heard of a guy having so much water in the tanks, that there wasn’t enough room left for sufficient air reserve.
     
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  9. spindrift

    spindrift Road Train Member

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    That boy gets a WOW. I pretrip. I could see it happening though.
     
    Last edited: Nov 9, 2021
  10. spindrift

    spindrift Road Train Member

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    Apparently, I have more troubleshooting to come. Wish I didn't need a two person crew. I'm used to workin by myself.
     
  11. Dino soar

    Dino soar Road Train Member

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    That's a no joke.

    I was having problems in my air system before I put my truck into service. I can't remember what exactly it was doing but it was really something weird.

    So I went to the tanks behind the wet tank and drain them. There was so much inside of those tanks, and it was all this wet oily thick Mucky stuff.

    I don't know that those tanks were rever drained even once. I had to keep draining them over and over and over and over and over again.

    Finally I got Pure Clean Air coming out and no more problem.
     
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