Rear Brake locking up

davejake

davejake

3 is company too!
My newly acquired 81 X1/9 has a strange brake problem (this car had not been driven for a few years). The right rear brake will lock up and when this happens the brake pedal is rock hard with no travel. Once the car sits for 30 minutes and cools down, the brake is fine again -- sometimes working fine for the rest of the 30 minute drive home.

We took off both rear calipers, removed, cleaned and greased the retainer clips to be certain that it is floating correctly. We also bled both rear calipers with a good 6-10 full depressions of the brake pedal. I didn't observe anything unusual, nor air bubbles coming out from the bleed.

What next? What could this be? This caliper has definitely been smoking hot. It was painted and now the paint is burned off and the brake pads looks very burnt. The piston did move and released after the remove/bleed mentioned above.
 
Next time it locks up, try cracking the bleeder open. Chances are that you'll get a spurt of brake fluid and then it will unlock for a while.

You can try replacing the hoses (they can collapse trapping fluid under pressure in the caliper) and rebuilding the offending rear caliper (a real pain because of the e-brake mechanism).

I'm not going straight to master cylinder replacement because only one of your two rear calipers is affected and the problem goes away when the car sits and rests. An MC problem would affect both rear calipers and generally doesn't clear itself.
 
davejake said:
My newly acquired 81 X1/9 has a strange brake problem (this car had not been driven for a few years). The right rear brake will lock up and when this happens the brake pedal is rock hard with no travel. Once the car sits for 30 minutes and cools down, the brake is fine again -- sometimes working fine for the rest of the 30 minute drive home.

We took off both rear calipers, removed, cleaned and greased the retainer clips to be certain that it is floating correctly. We also bled both rear calipers with a good 6-10 full depressions of the brake pedal. I didn't observe anything unusual, nor air bubbles coming out from the bleed.

What next? What could this be? This caliper has definitely been smoking hot. It was painted and now the paint is burned off and the brake pads looks very burnt. The piston did move and released after the remove/bleed mentioned above.
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It’s your flex hoses had that happen with my 83 too. 100% changed them and all was good! Was bled many times prior.
 
The hose could be bad. The hoses can get soft and collapse internally which blocks the fluid from moving.
 
I had the same symptoms with bad hoses. When they get old, they can become very constricted, but will still pass fluid due to the high pressure from the master cylinder. However, when that pressure is removed, it can take quite some time for the fluid to bleed back into the master cylinder through the constricted hose and release the caliper cylinder.
 
I went for a test drive in a Volvo several years ago; smoke curled up out of the front fenders from the brakes- got it for half the asking price and changed the hoses.
 
Thanks for all the tips! 4 new braided brake lines are ordered! How worried about the overheated caliper should I be. Can I remove and inspect the piston to make sure any rubber gasket is in tact? or if I remove it, do I automatically need to replace the o-rings?
 
davejake said:
How worried about the overheated caliper should I be. Can I remove and inspect the piston to make sure any rubber gasket is intact? or if I remove it, do I automatically need to replace the o-rings?
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You are right to be worried, but if it were my car I would hope for the best and not mess with the guts of the caliper yet. If it locks up again after hose replacement or other problems appear, you can go into the caliper then. That's my personal choice on the facts as presented and the tradeoff I make between bleeding the brakes one more time (no big deal) and rebuilding a rear caliper (something of a pain) and you may reasonably decide to be more aggressive once you're in there.

When you have the caliper off for the hose replacement, you'll be able to look at the rubber outer boot around the piston. That's an easy replacement that doesn't require opening up the caliper, so you'll want to do that if it is torn or damaged or even missing. If it is badly heat-damaged, then I might reconsider the hope for the best suggestion above.

Be aware that the rear calipers ride on a threaded shaft for the ebrake mechanism so unlike the fronts they can;t just be pressed in - you have to rotate them clockwise in and counterclockwise out. That's what the slot in the face of the piston is for. Also be aware that the brakes won't bleed properly unless that slot is up (it's off-center).
 

Thanks Eric. I have already seen that outer seal and it looked fine. I did know about the threaded piston, but didn't know the direction of the slot mattered! -- so, which direction is "up"!?!?
 
Eric had it right, one detail to add, on the rear caliper cylinders there is a small scribe line parallel to the slot. That line has to toward the bleeder, there is a recess machined in to the back of the piston that let's an air bubble out. Line does not point to the bleeder, just on that side of the piston.
 
Rear caliper rebuilds are only daunting when you haven't done them before.
I combined the advice I received from all here, and did a write up when I overhauled my brakes earlier this year.
Your welcome to use from it what you like. Dan S. calls it my "Rear Caliper Rebuild for Dummies" series.
 

Attachments

  • Rear Caliper Rebuild(rev4).pdf
    1.9 MB · Views: 216
Should change the pads too, if they got cooked. The rotors as well, if there is any pulsation in the pedal - since they will warp when overheated. I'd definitely follow Eric's advice & leave the calipers alone until after the hoses.
 
Tom Ginefra said:
Rear caliper rebuilds are only daunting when you haven't done them before.
I combined the advice I received from all here, and did a write up when I overhauled my brakes earlier this year.
Your welcome to use from it what you like. Dan S. calls it my "Rear Caliper Rebuild for Dummies" series.
Click to expand...

Thank you Tom for taking the time and effort to post this !! i really like the PDF format - I will print it off and insert into my workshop manual.
 
Thank you very much for this write up. I see is that there is a clip and other parts inside of the piston but these are not mentioned in the write up. Should I take from that that I should leave that alone and just shoot in some brake cleaner and wipe it clean? Thanks.
 
I have always left the inside of the piston as is.
Not very helpfull, but this is what extreme brake failure looks like.
Davison_brakefire_32.png
 
Michael Oxman said:
Thank you very much for this write up. I see is that there is a clip and other parts inside of the piston but these are not mentioned in the write up. Should I take from that that I should leave that alone and just shoot in some brake cleaner and wipe it clean? Thanks.
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If you're referring to the clip that holds the ratcheting mechanism in place... Yes. Unless there is something seriously wrong with that assembly, I would do just as you suggested, and not attempt to take it apart.
 
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