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. bjrc

    bjrc Light Load Member

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    Might take the hose loose from the intake manifold that supplies the air inlet to the compressor and ck for backflow out of it. Could be leaking back into the intake and may not be hear it.
     
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  3. spindrift

    spindrift Road Train Member

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    Is there a check valve in the compressor that prevents that from taking place?
     
  4. bjrc

    bjrc Light Load Member

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    Should have a one way ck valve that lets air flow to the compressor but not the other way.
     
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  5. bjrc

    bjrc Light Load Member

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    I dont know on your truck if it has a dedicated ck valve or uses the reed valves in the compressor itself.
     
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  6. SmallPackage

    SmallPackage Road Train Member

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    I’ve stripped a few trucks down for restorations before that have had this problem. The tanks will be HEAVY when removed and you instantly know fun times to clean them out lay ahead. My ‘51 Diamond T air tanks are like that. Wipe ‘em down with soapy water and they look like a soap stone rock in an aquarium. But completely shut down It still holds air for 6 hrs. and meets the other dot requirements for leak down.

    Funny story sorta related lol!
    A friend of a friend used to set the air in his pickup tires with a hose and chuck off of an old dump truck he drove for a big construction outfit in San Antonio because he parked it next to the truck at their yard and would leave it for days or weeks sometimes. After a few years he got a severe vibration at 5mph and beyond. Since I was in the tire business He asked me if I could ride along to see if it was tire related. Along with the horrible shake I noticed when he came to the complete stop the little truck would surge forward a little. Long story short. The tires had a few gallons of water and oily crud in them from that old dump trucks compressor.
     
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  7. Inderjit

    Inderjit Medium Load Member

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    I ran into a similar problem years with a cement truck. The truck came
    into the shop for its annual safety inspection. When I made a full
    brake application with 120 PSI in the reservoir the low air warning came
    on when the pressure dropped to 65 PSI. There was no air leak either.
    When the outfitter installed the drum and hydraulics they moved the air
    tanks to outside the frame to clear the drum drive and hydraulic pump
    and lines. The pump was driven by a live PTO off the rear of the engine.

    The truck had one compartmented air tank, a tank with approximately
    25% wet tank 75% secondary reservoir with an internal check valve
    feeding the secondary reservoir from the wet tank section.
    When they reconnected the air lines to the tanks they connected the air
    line that supplies air to the drive axle relay valve to the wet tank end of the
    secondary tank and not to the primary tank so there was only 25% of the
    reservoir volume to supply the rear brake chambers.

    The cement truck was twenty years old and had this problem from the
    day it went into service judging by the layers of cement built up on the tanks.
    It had been through over twenty annual safety inspections as well and no
    one ever addressed the problem.

    Has any work been done on your truck recently that would have required
    the removal of an air tank or disconnection of air lines from the air tanks?
     
    spindrift Thanks this.
  8. spindrift

    spindrift Road Train Member

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    Glad you were able to find the problem. Hard to imagine it went that long.

    No work of that kind done to my truck in the time I've owned it...
     
  9. lwlevens

    lwlevens Medium Load Member

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    Ever find the air leak???
     
  10. spindrift

    spindrift Road Train Member

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    No sir. Still workin' the problem.
     
  11. dibstr

    dibstr Road Train Member

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    Check your hand valve. If it leaks out of the exhaust port when you apply the foot valve get back to me.
     
    Last edited: Nov 19, 2021
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