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1993 FLD 120 brake issues, rear brakes don't seem to grab very hard.

Discussion in 'Trucks [ Eighteen Wheelers ]' started by KittyKicker, Jul 29, 2019.

  1. Jul 29, 2019 #1
    KittyKicker

    KittyKicker Light Load Member

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    I have a 1993 FLD 120. It is very hard to get the truck to stop. It feels like the front brakes are doing their job, but the rears don't seem to want to grab very hard. When I pull a trailer I can tell that most of the braking force is coming from the trailer. The trailer brakes respond normally while the drive brakes really don't feel like they are doing much of anything. I have replaced the foot valve, the relay valve, and all of the shoes and drums around the truck. Any ideas?
     
    KittyKicker, Jul 29, 2019
    KittyKicker, Jul 29, 2019
    #1
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  3. Jul 29, 2019 #2
    boneebone

    boneebone Road Train Member

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    Was the valve you replaced the Drive Axle Service Relay Valve?
     
    boneebone, Jul 29, 2019
    boneebone, Jul 29, 2019
    #2
  4. Jul 29, 2019 #3
    KittyKicker

    KittyKicker Light Load Member

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    I replaced both the foot valve and the relay valve.
     
    KittyKicker, Jul 29, 2019
    KittyKicker, Jul 29, 2019
    #3
  5. Jul 29, 2019 #4
    AModelCat

    AModelCat Road Train Member

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    A lot of older trucks I believe had a bobtail porportioning valve. From what I can remember its purpose is to reduce braking pressure to the drive axle brakes when bobtailing.

    I would imagine the valve probably receives an air signal from the trailer supply. If the porportioning valve were to stick, it would be possible to be stuck in bobtail mode all the time and have weak drive axle brakes.
     
    AModelCat, Jul 29, 2019
    AModelCat, Jul 29, 2019
    #4
    KittyKicker, singlescrewshaker and BoxCarKidd Thank this.
  7. Jul 29, 2019 #5
    BoxCarKidd

    BoxCarKidd Road Train Member

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    Also I have been running into a lot of problems with the incorrect shoes lately. Even if the unit always had Standard q's ( 4515.s? ) It may need Q + ( 4707's? ) after the s cams are replaced. There are gauges to measure the cams and rivet spacing in order to match them up.
    The rule that standard Q s have a hump by the big spring pin has been broken. Watch out.
    Even Pro Par shoes in the mix sometimes and Eatons have multiple shoes as well.
    Connect a gauge to a chamber service hose to check what AModel said.
     
    BoxCarKidd, Jul 29, 2019
    BoxCarKidd, Jul 29, 2019
    #5
    KittyKicker and spsauerland Thank this.
  8. Jul 30, 2019 #6
    KittyKicker

    KittyKicker Light Load Member

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    KittyKicker, Jul 30, 2019
    KittyKicker, Jul 30, 2019
    #6
  9. Jul 30, 2019 #7
    KittyKicker

    KittyKicker Light Load Member

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    I appreciate it. I will definitely check that out.
     
    KittyKicker, Jul 30, 2019
    KittyKicker, Jul 30, 2019
    #7
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