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. spindrift

    spindrift Road Train Member

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    I've got something strange going on with my air. When coming to a stop or braking relatively hard, I'm losing air pressure at an alarming rate. Stopped and asked a guy to depress the brake pedal while I listened to all six of the brake chambers and no leaks there. Maybe the air governor that sits on the drier? I put a new one on last year. I don't want to just throw parts at it but since those things are pretty inexpensive, maybe I just go ahead and change it?

    Losing pressure bobtail as well.
     
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  3. skallagrime

    skallagrime Road Train Member

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    Im Going to guess the tractor brakes werent released when you had him push the brakes, or he barely touched the pedal.

    Chock wheels
    Build air
    Release tractor brakes
    Turn truck off
    Wedge the pedal to 40-60 psi (really pushing the pedal)

    Take your walkaround again
    Im still betting its a brake chamber or hose

    2nd option, the leak is the hose/tractor release valve itself and air loss is masking that air loss is happening there

    3rd option is the brake pedal valve/hose itself, but you should have heard that when you were at the drivers side steer
     
  4. spindrift

    spindrift Road Train Member

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    I checked again and confirmed it's not #1 or #3. I don't hear any air loss while it's occurring so I'm not sure how I would identify the issue if it was #2.
     
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  5. Dino soar

    Dino soar Road Train Member

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    If the air governor cuts in and you build the air until it kicks off your Governor is working correctly.

    Just a thought but if you have something else that's leaking on the truck when you hit your brakes it will look more dramatic. Have you tried just shutting the truck down and seeing if you're losing air without hitting the brakes?

    Some trucks the blend doors and other things are air actuated and you could have a leak somewhere else maybe.

    Also I would build to full pressure and release the brakes and jam the paddle down with something and listen at the treadle valve and all the brake relay valves to see if you have any leak there (chock wheels).

    If all else fails get the ultrasonic leak detector. I've been meaning to get one and now that I'm writing this I think I'll order it right away.
     
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  6. skallagrime

    skallagrime Road Train Member

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    You say "ALARMING RATE"

    What *exactly* does that mean?

    We can rule out a brake chamber simply because its not getting progressively worse with time. Pancake failure usually progresses pretty fast.

    As dino mentions, the brake relays or hoses under truck back to where they split is probably next on the agenda if you can get rule out the brake valve assembly itself
     
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  7. spindrift

    spindrift Road Train Member

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    Just before I turn onto the road that leads home, I go down a hill that's approximately a half mile long before I have to stop at a traffic light. Depending upon how fast I come down that hill bobtail, while sitting at the traffic light to make my turn, foot on the brake because there's still more hill beyond the light, my air pressure alarm can activate while sitting and waiting for the light to turn green.
     
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  8. skallagrime

    skallagrime Road Train Member

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    Soooo greater than 1 psi loss per second.

    And you can hear nothing anywhere while braking or when stopped?

    No air hiss?

    If thats the case, its shop time id wager
     
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  9. Rideandrepair

    Rideandrepair Road Train Member

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    How old are you OP? Maybe you’re just hard of hearing?
     
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  10. spindrift

    spindrift Road Train Member

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    Brutal, just brutal.
     
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  11. Rideandrepair

    Rideandrepair Road Train Member

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    If it’s at home now, maybe ask the Little Lady to hold the brakes, or chalk the tires, put a 2x4 between the seat and pedal, parking brakes released, shut it off. Listen close, spray it with the water hose. I suspect the foot pedal or one of the relay valves.
     
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