1998 Peterbilt 379 brakes, what is normal when pushing the pedal?

Discussion in 'Peterbilt Forum' started by drewfarmsllc, Jan 31, 2021.

  1. drewfarmsllc

    drewfarmsllc Light Load Member

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    I have a 98 379 and the brakes are fair at best. I put new slack adj on it 2 years ago and new pads last week. It will stop but you have to put a lot of pressure on the pedal. It has a guage that will go to 60 psi when I press as much as pedal will go
    Is this normal? Any tips?
     
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  3. SmallPackage

    SmallPackage Road Train Member

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    That is not normal. Shouldn’t need more than 10psi application pressure to stop normally if adjusted properly. If brakes worked better before new shoes were installed my first guess would be the wrong “series” shoes were installed and/or drums are out of spec on max wear diameter allowed.
     
  4. Don379

    Don379 Heavy Load Member

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    I also have 379. At full brake pressure, that gauge should spike to however many psi you have on your primary and secondary tanks psi. So if you have 120psi in your tanks, at full brake application you should get same reading on your brake psi gauge. You have an old truck, probably in need of a new foot Valve. I agree with the other guy also, your truck should be stopping with 60 psi. New slack adjusters and brakes won’t fix the issue if your s cams and bushings are wore out. Especially if they are original like mine were that I just replaced. Check out my thread that has pics of that issue
    Uneven brake wear and cam bushings
     
  5. AModelCat

    AModelCat Road Train Member

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    Pads?

    These disc brakes?
     
  6. Don379

    Don379 Heavy Load Member

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    He might be green buddy. He means shoes. Being since it’s a 98
     
  7. AModelCat

    AModelCat Road Train Member

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    Could be. They've had disc brakes back in the 80's though. I thought I read disc brake in the title when I clicked it so maybe OP changed the title? Just wanna be on the right page.
     
  8. Don379

    Don379 Heavy Load Member

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    Here’s an easy check for you for the s cams. Chock your wheels or put in gear on a level surface. Shut the truck off and release your parking brakes. Go back to the rear tandems and put a pry bar under your slack adjusters and pry up and down. If that slack adjuster is all sloppy and moving the cam all around, they are shot and your truck isn’t gonna stop worth a dam.
     
  9. Don379

    Don379 Heavy Load Member

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    True but I highly doubt it. People refer to shoes as pads all the time. Just a slip of the tongue I’m sure. But hey maybe he will clarify. I don’t mind being wrong
     
  10. Joyce's Volvo

    Joyce's Volvo Light Load Member

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    Could be threadle valve ( foot) or could be relay valve, don is correct , you're on them hard - app gauge should match the tank psi immediately , if not , more than likely , foot valve.
     
    Don379 Thanks this.
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