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  • Way off topic truck issue

    I know there are a lot of car guys here.
    So I was driving home in the snow and the rear end on my sierra slid out and caught a curb pretty hard. For some reason this has caused my brakes to not work correctly. The pedal almost goes to the floor before the brakes kick in. It almost feels like the power brakes aren't kicking in. I tried disabling abs but it didn't have any affect. The brakes worked perfectly fine until I hit the curb.
    Also I did check the fluid and it is fine so I didn't bust a line. Could this be caused by a sensor?
    Any thoughts would be appreciated.

  • #2
    I'm no expert for sure, but I'd check, or look for information about the proportioning valve, and see if there is a mechanism that can move, and alter the pressure to the front vs rear brakes.

    They make a pressure gauge that can be connected to measure the pressure at the brake cylinder at each wheel. That may be a way to track down the problem.

    Look at the brake caliper, and see if it might have cracked, which would prevent it from applying force to the discs.

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    • #3
      ABS/Traction only interrupts the base brake system when there is a ABS/Trac event, otherwise the base brake system is passed thru the ABS Module without any change. Your problem is with the base brake system. Most likely a ruptured Caliper seal (w/disk) or a Wheel Cylinder (w/drum) or broken parts that are not anchored. There may (eventually) be brake fluid evident around the inside of that wheel.
      If you wacked it that hard, the wheel may be bent and/or the axle shaft as well. (you might feel the back end shimmy side-to-side especially when turning)

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      • #4
        My abs in my car didn't work for years. Didn't effect normal driving. I actually removed the system.

        If pedal goes to floor, you have a leak. If there is no obvious leakage in lines, calipers, then check master cylinder.
        If it's 1996 or newer, it should have an abs light and the ability to have codes checked with a scanner very easily. Go to a local airports store and have codes scanned for free. I am confident a bad sensor will not make brakes go to floor. Codes won't help with leak.

        Do you have rear discs?
        If you have drums, the wheel cylinders often leak bad when old.

        Check for rust. Maybe there was a rusty spot in metal line and impact cracked it. Damaged rubber brake hose?

        If no leak, i am guessing master cylinder.
        They often leak behind the cylinder where it meets booster. You will see what spot under it and paint missing and possible rust from fluid eating away at paint on booster.



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        • #5
          Been doing some troubleshooting. The brake booster seems fine. I would think the master cylinder is good too as I can get the brakes to engage fully but only just before the brakes hit the floor. I don't seem to have any leaks either. No fluid missing and no leaky spots in the lines or brakes.
          I doubt anything was broken or bent. The rim is fine except a little paint missing along the very edge where the bead meets And nothing feels wobbly. Hard to tell in the snow though.

          I have been doing some research and suspect it might be the abs control module. Apparently it can fail and not throw a warning. It has the same symptoms as I am having. I will check the master cylinder and pressure to brakes first though.

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          • #6
            Try bleeding the system. Not sure why an impact would cause low pedal without bent rotor or axle, but try simple stuff first. Also, complete brake inspection is in order. Safety first!

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            • #7
              Originally posted by mgodson View Post
              Try bleeding the system. Not sure why an impact would cause low pedal without bent rotor or axle, but try simple stuff first. Also, complete brake inspection is in order. Safety first!
              I will check the brakes tomorrow.
              The impact was on the rear (drum brake). I don't really see how air could have gotten into the system? It doesn't appear to be a leak as the fluid level has not lowered.

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              • #8
                Was the ABS active when it hit ? That could effect things. if it wasn't active at the time. Then I'd be more inclined to look elsewhere, the ABS system is pretty passive and reliable most of the time.

                If the system took in air some where, then the pedal would be soft but able to pump up. - With the key off, push the brakes several times to remove the booster's stored vacuum, (removing the boosters effect on the system). Then push, release, and quickly push again hard. Hold firm and steady. The pedal should gain and hold (more-or-less) solid. If the pedal "falls" from your foot with the same amount of "foot-force" being applied, Then the fluid (pressure) is going somewhere, either out or around internal seals.
                With an external leak, the pedal would fall pretty evenly, with little change in rate. Just like a garden hose, more pressure doesn't necessarily equal more flow, depends on the size of the leak.
                If the pressure is being bled around internal seals. Then the pedal will catch and hold solidly, then it will fall quickly. More "foot-force" might get the pedal feel back. (Inside, the umbrella seals will flair out against the fluid and hold pressure, Then the faulty seal will bleed off the high pressure in front too the low pressure behind it and the pedal falls with little resistance. Quickly applying more force, pushes the umbrella back out again)

                It's possible that the ABS Module has air in it or fluid is leaking past the internal seals. The module doesn't know what is in it, it just knows that it can (or can't) turn on and off the valves and the motor. A faulty wheel speed sensor will set a code and have nothing to do with the pedal feel.

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                • #9
                  Sounds to me like you popped a rear shoe. There's no reason to speculate on any part of the ABS system until you actually physically inspect the rear brakes. Don't forget to check both sides of the truck and make sure that the all of the retainers are still there

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                  • #10
                    +1, that's the most likely reason for the brake pedal going all the way down with a full master cylinder. The wheel cylinder where the shoe is popped is going all the way out without the shoes engaging the drum. But it shouldn't be as bad as what the OP says, since there are dual systems, and the popped shoe would only affect one. A complete examination of the brakes is in order.
                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

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                    • #11
                      +2 ... you're gonna have to take those suckers apart to see.
                      Electronics engineer, woofer enthusiast, and musician.
                      Wogg Music
                      Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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                      • #12
                        Try your parking brake. If it works as usual, don't know, but if it doesn't, there's something mechanical out of wack in the rear brakes.

                        Don't rely on the parking brake till you check!

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