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> Jensen Healey & Jensen GT Tech > Brakes > Brake judder due to sticking piston

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subwoofer
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My left front brake judders under braking (hard or loose, no difference). I suspected a warped disc, so I measured it with a dial indicator, runout less than +-.002". So that wasn't the reason. I then removed the brake pads, having a hard time getting the inner one loose since the piston wasn't retracting very easily.

I then tried jogging the piston a bit, and it seems it goes out easily, but is harder to get in than the other side. Did I do something wrong when I rebuilt the caliper? Is there an inner and an outer direction for the rubber seals? I used new pistons and a fresh TRW rebuild kit when I rebuilt it.

I am pretty sure I have found the culprit, since after the jogging of the piston, the brakes were fine for about 3 stops, then the judder came back. I also retightened all the bolt in the suspension, as I had painted the parts before assembly.

Any tricks up your sleeves, Gentlemen?

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Joachim

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I am suspect bearings since I get bad vibration on the front left corner under braking - left front locks up early, and also a bad shimmy in the 45-55MPH speed range. The disk is true (measured), bushings are new (SuperPro), dampers are new (Bilstein), both ball joints are new, steering ball joint is new. I have also tried swapping the left front and rear wheels to eliminate imbalance as a cause, and the caliper has been rebuild, then cleaned out again without improvement.

Opinions, anyone? Anything else I should try? This problem is driving me nuts ATM.

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Joachim

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Well,it definitely sounds suspension related. The shimmy makes it sound like it's not caliper related. But then....

 

I'd double check the alignment and wheel balance. Everything else already considered, I'd then go for the bearings. I would also check the vertical link for damage to it or the spindle.

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Typical faults on the vertical link?

Common sense suggests to me that the cause of the problem should be in or holding a rotating part. This is why I swapped front and rear tyres (tyres are directional). The car has probably been in storage for a long time before I got it, and I have not changed the tyres. Could it be that both tyres have an equal size flat spot, and that the two others haven't? The odds are too tall, I think.

At the recent EU-checkout (the Norwegian twist on the MOT), it was noted that the left front brake was uneven, wobbling between 50 and 150% of the other side, and this goes against a mass imbalance problem. The brake testers at the garage goes something like one revolution of the wheel every second or two.

I would imagine that if the caster angle was badly off (too steep), the wheel would wobble like the castering wheels on a shopping trolley, but I think it is fairly equal on both sides (count of threads on radius rods), and it doesn't make sense at the brake tester speeds.

I'm still stumped...

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Joachim

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What was wobbling during the test? the brake disk or the wheel?

 

Jody

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Brake power/balance. You know the kind of tester where you place both wheels of an axle on a set of rollers. The rollers start rolling and you press the brake pedal. The instrument has two dials showing the brake power on each wheel. The needle for the right wheel was steady at 100, the left needle wobbled up and down as the wheel turned.

Could not see any wobble on the wheel as such.
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Joachim

Last edited on 08-29-2010 04:02 pm by subwoofer

Brett Gibson JH5 20497
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what did the tester say was the common issue for seeing pulsating readings, sounds like a warped rotor to me.

Brett

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He didn't say... I too suspected rotor, but as I said, I have measured it and it was well within tolerances when just spinning it. The rotor has about 1000 miles on it from new.

Could it be that some of the rollers in the bearing is more worn than the rest, so the problem only shows under load? By pulling/twisting on the hub I could increase the runout to roughly twice the amount of the free spinning runout (cold bearings), but I have not done that on the good side for reference yet.

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Joachim

Dakota123
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Sounds exactly like a problem I had with an old Ford pick-up truck -- the caliper wasn't releasing completely causing the judder after just a short time driving..  Disk was true, but nonetheless a noticeable judder.  You mention a somewhat sticking piston in your first post...

Mike

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I pulled the caliper apart again last week, and I think it is fine now. I could push the pads away with my fingers again, but it is still as bad as ever.

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Joachim

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I think I am getting there! Changed the wheel bearings on the offending side tonight, and the shimmy seems to be gone. Still a little bit of brake judder left, but I discovered that I probably didn't tighten the bearings enough. A good deal of grease in the, I suspect it moved a bit when it got some movement in it. I could just feel a little bit of play after I came back from a 10 minute test drive.

How hard should it be to turn the hub when tightening with a wrench - before releasing again and setting the nut by hand? Spin the hub before setting the nut, or just tighten - release - set? At least I know the bearings are fresh, and the cups are properly set in the hub.

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Joachim

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Spin the assembly while tightening, then back off. There will be slight drag from the grease; it won't spin all that freely. (Caliper adds even more if installed.) Any more than about 1/8" tire movement top and bottom when the whole thing is given a shake means something isn't right. Ideally there should be essentially none as long as the bearing does not run hot.


I seem to remember a board member having.a problem keeping the races in position. The hub was oversized, I think it was.

Mike

Last edited on 09-03-2010 10:21 pm by Dakota123

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Brake judder still present, shimmy is almost gone after the new bearings. I remember that there was some pitting (size of a medium sized coin) in the inner bore of the failing caliper, this is probably the reason for the problems. It is time for a new caliper, methinks...

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I had one last go at the caliper tonight, I really wanted the brakes to work properly before going to Denmark next weekend, and I struck out at obtaining another caliper in time.

Had the caliper off and the pistons out. Then I attacked the pitted areas with 150 and 400 grit sandpaper, trying to smooth the area as much as possible. Then I put in new rubber piston-to-bore seals for good measure, and jogged the pistons ten-fifteen times after bleeding.

Finally, I have brakes that don't judder (much). There still a very small amount left, but that is down to a worn steering rack and not perfectly balanced wheels, methinks. Oh joy!

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Joachim



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