Can a bent knuckle cause brake locking? - need expert advice on this one
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Can a bent knuckle cause brake locking? - need expert advice on this one
Hi there,
I've been trying to solve a truly stubborn brake problem. Symptoms are left front wheel running 10-20 degrees F. hotter than others (as measured by aftermarket TPMS), left front locking up during panic stop with ABS working correctly with other three wheels, and the pedal not returning even with no foot pressure on it during panic stop until vehicle comes to complete stop. If I go to a gravel parking lot, build up speed and jam on the brakes, the ABS activates and the pedal stays low and doesn't release until the vehicle has completely stopped. I've been through several left front calipers, replaced all flexible hoses, master cylinder, HCU, proportioning valve, and today, the booster. New fluid, multiple pressure bleedings. None of these replacements have changed the symptoms. It has now destroyed two left calipers, once by cracking the caliper casting itself and most recently, today, by cooking/cracking the phenolic piston. Having replaced everything hydraulic, today I started to suspect the underlying problem is mechanical. The brake pad mounts on which the pads slide are, on this vehicle, part of the knuckle--they can't be replaced on their own. I started to wonder if the pad mounts could be bent or corroded and therefor not completely parallel to each other. Sure enough, when I measured the distance between the two pad mounting surfaces on the knuckle, the distance between the two mounting surfaces was about 1/32-1/16 inch greater for the outer brake pad than for the inner brake pad. I checked this on the right knuckle, and found no such difference. So here is my question: has anyone seen this problem before, where a bent or corroded pad support causes the pad to fit in too loosely, and then this causes brake dragging/locking? The fact that two left front calipers in a row have failed mechanically suggests that something basic is wrong. Having I finally have gotten to the root of the problem? Thanks so much for advice/help. This has been crazy making.
I've been trying to solve a truly stubborn brake problem. Symptoms are left front wheel running 10-20 degrees F. hotter than others (as measured by aftermarket TPMS), left front locking up during panic stop with ABS working correctly with other three wheels, and the pedal not returning even with no foot pressure on it during panic stop until vehicle comes to complete stop. If I go to a gravel parking lot, build up speed and jam on the brakes, the ABS activates and the pedal stays low and doesn't release until the vehicle has completely stopped. I've been through several left front calipers, replaced all flexible hoses, master cylinder, HCU, proportioning valve, and today, the booster. New fluid, multiple pressure bleedings. None of these replacements have changed the symptoms. It has now destroyed two left calipers, once by cracking the caliper casting itself and most recently, today, by cooking/cracking the phenolic piston. Having replaced everything hydraulic, today I started to suspect the underlying problem is mechanical. The brake pad mounts on which the pads slide are, on this vehicle, part of the knuckle--they can't be replaced on their own. I started to wonder if the pad mounts could be bent or corroded and therefor not completely parallel to each other. Sure enough, when I measured the distance between the two pad mounting surfaces on the knuckle, the distance between the two mounting surfaces was about 1/32-1/16 inch greater for the outer brake pad than for the inner brake pad. I checked this on the right knuckle, and found no such difference. So here is my question: has anyone seen this problem before, where a bent or corroded pad support causes the pad to fit in too loosely, and then this causes brake dragging/locking? The fact that two left front calipers in a row have failed mechanically suggests that something basic is wrong. Having I finally have gotten to the root of the problem? Thanks so much for advice/help. This has been crazy making.
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Sounds like a possibility. If the pads are either loose or getting wedged into place it will cause them to drag or float up against the rotor. Would it also be possible that the bearing or an alignment issue is causing that corner to get hotter and make the brakes sticky? Just a thought.
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Thanks for the feedback - how much slop is too much?
Grswat,
For the pad with the most slop, the outer one on the left front, I'd say the total clearance between the metal of the pad and the upper and lower mounting surfaces is somewhere between 1/16 and 1/8, with something like half that amount on the right front, which is having no problems. Do you think that is enough to cause severe binding/grabbing? I've thrown so many parts at this I want to be as sure as I can before I replace the knuckle. The bearing was also replaced in the last few months, and the van moves down the road straight.
For the pad with the most slop, the outer one on the left front, I'd say the total clearance between the metal of the pad and the upper and lower mounting surfaces is somewhere between 1/16 and 1/8, with something like half that amount on the right front, which is having no problems. Do you think that is enough to cause severe binding/grabbing? I've thrown so many parts at this I want to be as sure as I can before I replace the knuckle. The bearing was also replaced in the last few months, and the van moves down the road straight.