Stuck caliper pistons - retracted

I searched the forum for posts about stuck caliper pistons in the retracted position and nothing came up. So I am asking a fundamental question here: after rebuilding my front brake caliper (cleaned, replaced seals and pads, bled, etc.) the pistons do not move. They are stuck in the retracted position which I put them in so I could get the new pads in place. My specific question is while they are stuck shouldn't my brake lever be hard to pull? After hours of bleeding a new master cylinder and new braided steel lines I still have no pressure. Yet the pistons remain retracted.

Anybody have a guess as to what the problem is? I am about to remove the caliper and start over.

Thanks in advance!

Peter

Keep bleeding, they'll come out.

concours said:

Keep bleeding, they'll come out.

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Really? Is this typical of bleeding a system that starts out dry?

From one Peter to another we are always looking for extension not retraction on those pistons. It's very possible you have retracted so much to final seating home that the little brake fillup caliper cavity holes are blocked by the pistons themselves. Undo line and whack on wood chunk. How much wood could a woodchuck chuck if a woodchuck could chuck wood ?

Ha ha hahhh! That's good. Yeah, I wondered that myself. I was thinking about applying heat to the caliper. Maybe I'll start over and not be so ambitious about pushing those pistons back next time.

you only want the piston stuck in there enough so that you can slide the disc between on install

and yes, you sure can put them in too far as in blocking the hydraulic fluid pressure

Undo the metal pipe from the caliper and give it a bit of a blast with compressed air. That should move the pistons.

Mark F said:

Undo the metal pipe from the caliper and give it a bit of a blast with compressed air. That should move the pistons.

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Hmm - I might try that this afternoon. I would rather avoid taking the caliper off if I can help it.

Everyone's input has been helpful - THANKS!

Stuck rusted or too far in caliper pucks are a long traditional infamous task that some including me have drilled and tapped the back side of the caliper to knock out the inner puck with a drift * after drilling and tapping the front puck to screw in a bolt to pull on if the stuck front puck'n just didn't come out with the tap drill pull. That of course was "after" the terrific fight on my 1st stuck puck'ns Lockheed > heating and banging for days to incrementally move it by inertia alone with a stack of super magnets on puck to add some slam-bam mass to em. Then I rebuilt Ms Peel with SS pucks and as mental emotional stress preventive for me or the next fella I drilled and tapped an access hole afterwards in Peel's, so never again will caliper service be any strain even with the 'easy' to move SS pucks. Set screw seals the access hole for the long run. I rebuilt Trixie with factory steel slightly rusted pucks as was able to knock them out but if next time they resist even a little beyond a few min. of heated slam-bam its off to the machinist for about instant relief.

Not much progress tonight. Let me clarify a couple of things. I was able to remove the pistons with no more than the expected hassle. I had them buffed in a machine shop once I saw that the tar and corrosion was not coming off with light steel wool. I refit them with new seals and a coating of Permatex ceramic lubricant. They were difficult to push back, but with a steel bar and my body weight I got them pushed back enough to insert the brake pads.

Another major change was installing a Brembo 13mm master cylinder and braided steel brake lines. I do see flow during the bleeding process so I believe the new master cylinder is working.

So as of tonight I have been bleeding air out of the system over 4 or 5 sessions, getting a little bit more air out each time. But the pistons have NOT moved at all! Unfortunately I don't have access to compressed air other than a tire pump. I figured I would hook up the syringe I've been using to do reverse bleeding and see if i can move them with that, but the tubing would probably pop off before the pistons would budge.

My question now is should I suspect the master cylinder? There has been no hydraulic pressure since i started this project.

Thanks in advance,
Peter

I had much the same problem with the rear calipers on my bike. I ended up using a rubber tipped air blower and pushed them out with a blast from the compressor. Then once the pads were extended against the rotor, I reattached the lines and used a reverse bleeding method to fill up the system (pushed fluid in from the caliper end).

Image of the sort I used:

Sounds like it would help a lot. But what If I don't have access to a compressor? Or maybe I should ask, what kind of compressor did you use?

Contours,

After reading your recent post describing what you did, I am a little confused. Did you say after you had the pistons buffed they would not go in? Were the caliper bores clean and smooth? The pistons should move in the bores without resistance other than the rubbers. They are essentially hydraulic cylinders. Bleeder or hose should be open when assembling to release air behind piston during assembly...this is usually done on the bench before assembling to the bike. Bore and rubber should have some brake fluid applied to lubricate the parts when you put them together. The amount of force you said you applied is not normal.

dennisgb said:

Contours,

Did you say after you had the pistons buffed they would not go in? Were the caliper bores clean and smooth? The pistons should move in the bores without resistance other than the rubbers. They are essentially hydraulic cylinders. Bleeder or hose should be open when assembling to release air behind piston during assembly...this is usually done on the bench before assembling to the bike. Bore and rubber should have some brake fluid applied to lubricate the parts when you put them together. The amount of force you said you applied is not normal.

Click to expand...

Yes, everything seemed clean. I used grease instead of brake fluid to lubricate the seals. And yes, I did think it required too much force to move them back. After today's lack of success I guess I'm going to pull the caliper off and free up those pistons. At least I have the bleeding process down pat now.

contours said:

dennisgb said:

Contours,

Did you say after you had the pistons buffed they would not go in? Were the caliper bores clean and smooth? The pistons should move in the bores without resistance other than the rubbers. They are essentially hydraulic cylinders. Bleeder or hose should be open when assembling to release air behind piston during assembly...this is usually done on the bench before assembling to the bike. Bore and rubber should have some brake fluid applied to lubricate the parts when you put them together. The amount of force you said you applied is not normal.

Click to expand...

Yes, everything seemed clean. I used grease instead of brake fluid to lubricate the seals. And yes, I did think it required too much force to move them back. After today's lack of success I guess I'm going to pull the caliper off and free up those pistons. At least I have the bleeding process down pat now.

Click to expand...

I hope you meant to type "SILICONE grease instead of brake fluid".... if you used petrol based grease, you get to do it over again, new seals.
It would swell the seals.

So true. Disconnect caliper. Bench dis-assembly time . Order in new seal kit. Brake fliud assembly only. Buffing or polishing only for external appearance, never internal workages. Plus -the new seals have an internal taper and must be installed as specified. Stainless pistons never steel for such a rebuild ,but you know.

contours said:

dennisgb said:

Yes, everything seemed clean. I used grease instead of brake fluid to lubricate the seals. And yes, I did think it required too much force to move them back. After today's lack of success I guess I'm going to pull the caliper off and free up those pistons. At least I have the bleeding process down pat now.

Click to expand...

Click to expand...

If you used petroleum based grease you need to start over as mentioned previously. You should test the fit of the pistons in the bores without the seals. They should slide freely. If not there is something wrong. Also, it is easy to cock the pistons in the bores if you pry on the sides of them. I use a c-clamp on the center of the piston or on the center of a piece of steel that bridges across the diameter of the piston in order to get equal force on the piston. This is usually only needed when moving pistons to change pads, not after a caliper rebuild, but it still would work to move pistons back. Most times on rebuilt calipers you can push the piston(s) back with your fingers.

Torontonian said:

Plus -the new seals have an internal taper and must be installed as specified.

Click to expand...

Now this I did not know. Obviously I paid no attention to the taper - didn't notice it either. I had instructions, but there was no mention of orienting the seals.

dennisgb said:

If you used petroleum based grease you need to start over as mentioned previously. You should test the fit of the pistons in the bores without the seals. They should slide freely. If not there is something wrong. Also, it is easy to cock the pistons in the bores if you pry on the sides of them. I use a c-clamp on the center of the piston or on the center of a piece of steel that bridges across the diameter of the piston in order to get equal force on the piston. This is usually only needed when moving pistons to change pads, not after a caliper rebuild, but it still would work to move pistons back. Most times on rebuilt calipers you can push the piston(s) back with your fingers.

Click to expand...

I used a ceramic based grease. The label said it was okay for seals. The pistons did slide freely without the seals and went in easily once the seals were in, but they would not slide back far enough to get the pads in. I did use a flat bar that was almost the same width as the pistons. It's puzzling.

I'm going out of town this weekend. I'll pull the caliper off and take it with me since I have a friend who owns a dive shop where I am going. I'm sure he'll have compressed air (in addition to what he uses to fill tanks).

Might want to do the blow job under water in a bucket to displace air inside so nil expansive projectile force when pushed out.