Can "seized caliper" pistons still move?

gsxrken · gsxrken's Garage

I just completed installing our host's braided SS flexible brake lines and a fluid change, and after a full bleeding session, I noticed I can hear a creak/creak/creak kind of sound as I press increasingly harder on the brake pedal. It's definitely coming from the FR RH caliper of my '86 930. Figuring that perhaps I had a "seized caliper", I took it off and was able to draw both pads and pistons back to the caliper body with a c-clamp. I then re-installed it, and with a few brake pedal pumps was able to see that the pads moved back to both sides of the rotor.

So, can I determine that since my pistons all move in and out, that they are not seized? Sounds simple enough, but I was wondering if the c-clamp would over-power anything and that "seized" can also mean "doesn't move easily" or "sticky"...

Since the car doesn't noticeably pull on braking, I don't want to spend the $104 that the turbo caliper seals presently go for from our host if I don't need to. But I am still wondering WTH the noise is. Has anyone else ever noticed this, or could try it in their garage with the car off and report back whether or not they hear anything sound like it's undergoing strain?

Flieger · Flieger's Garage

What you have are "sticky" pistons, sometimes caused by infrequent use, but always by moisture in the fluid (unavoidable except through frequent brake fluid bleed/flushing).

The fix is to "exercise" the pistons, in the way that you just did. You might now want to bleed the fluid, or do a partial flush.

hasaramat · 09-12-2010, 08:15 PM

I don't know the answer to your brake problem but I do know who might be able to help you. Eric Shea at PMB performance has a lot of brake knowledge and is a great guy when you need help with your brakes. Brake Restoration Service

Flieger · Flieger's Garage

+1 on Eric

gsxrken · gsxrken's Garage

thanks for the replies and link. I'll drive it a bit and see if it works itself out.

DanielDudley · 09-13-2010, 04:39 AM

Jack up the front wheels. Push hard on the brakes. Spin wheels. Wheels should spin freely, with little drag. If they don't, either the pistons are binding or the pads. Pads can bind on corrosion in the calipers, but usually it is the pistons. Make sure your rotors aren't flopping due to loose or worn bearings, and check to see that the calipers are secure on the car. Get rid of any buildup where the pads are inserted, and don't suck up the dust.

Binding pistons can make the brakes drag, and cause rapid pad wear, hot rotors, and uneven braking. Rebuilding calipers is basic removal of the calipers, pulling the pistons, and cleaning. Assembly consists of replacing the seals and boots, with a little silicon brake grease applied to the piston and bore.

Rebuilding is much cheaper than buying reconditioned units, but if the pistons are pitted or corroded, you will want to replace them. You are well qualified to do this job, and there are many tutorials on how to use compressed air to extract the pistons. I often pull the pads and use the brake pedal to push the pistons out myself.

You are well qualified to do this job.

T77911S · 09-13-2010, 04:43 AM

use the brakes...HARD. i think brakes are one of the most underused things on a car. that is why pistons get stuck. i sometimes find myself barely touching the brakes to slow down, which i dont think is good to do all the time.

the BMW i just bought had a rock hard pedal, you would think the brakes were awesome. i almost ran out of road going to work when i first got it. there is this short road i go down, brake hard and make a left, hard on the gas then hard on the brakes to stop before run into the gate to the tower. i hit the brakes to make that left and it was as if the brakes were massively overheated, i barely slowed . i almost did not make the left. my point, the calipers where so stiff form little use, they were not there when i needed them.

i removed the pads and pushed the pistons in and out a few times. to my surprise, they were not stuck, but a bit stiff. now since i use the brakes very hard going to work every day, they have gotten much better, almost to the point i am trusting them.

oh, the engine is running much better since it sees some upper RPM's. not hard ones, i may hold it at 4-4500 if i see that i am going to just slow down again.

DanielDudley · 09-13-2010, 04:48 AM

BTW, you can probably put 5000 psi through a brake pedal in a panic stop. You can always get a piston to move toward the pad even if it is partially siezed. It is the flexing of the piston seal that causes the piston to retract. That is virtually no retraction force at all. Pads often free themselves by wearing away from the rotor rather than retracting, if the calipers are sticking. But every time you hit the brakes, they will have to wear free again.

At best you will have hot brakes and uneven pad wear. Worst case, you will smoke your rotors.

DanielDudley · 09-13-2010, 04:55 AM

Quote:

Originally Posted by T77911S

i removed the pads and pushed the pistons in and out a few times. to my surprise, they were not stuck, but a bit stiff. now since i use the brakes very hard going to work every day, they have gotten much better, almost to the point i am trusting them.

I have seen guys move the pistons out and clean the crud off them just pulling the dust boot and not rebuilding the caliper. Sometimes it works. If there is crud on the pistons when you retract them to replace the pads, you will often get binding that you didn't have before you replaced the pads. The tolerance between the piston and the bore is very tight.

T77911S · 09-13-2010, 05:38 AM

yes, usually the rubber boots are torn or will tear trying to get the piston back in.

when i bought my 911, one piston on each caliper was seized. going from 914 brakes to 911 brakes with a seized piston was about the same. thats all i could do at the time, unseize the piston and put them back on. WOW! what a difference. i still have not had time to go back and rebuild the pistons.
BTW, for the OP, a seixed piston does not move and usually requires some thought to get it lose. like putting the caliper back on, put the new pad on the good side and stomping the brake pedal to break it free. what happens with dual piston calipers is one side will start to do all the work, the other side corrodes or just gets stuck, thats why i suggest using the brakes hard enough so both sides have to do work.

i spent several hours on a friends volvo because of stuck pistons, it should have taken less than 20 minutes to put pads on a volvo, btw, he drives like an old lady, wrecks and all.