Brake pistons stick after warm up

[meichten]

I’m having a brake issue, have an 80 SC, rebuilt rear calipers just put on, new ATE MC, all steel brake lines about 10 years ago from PO. Bled the lines after putting in the new hardware. I can go out for a drive, however after about 5-10 minutes the pistons are not releasing. Get home and bleed one of the calipers and they are able to release and fine for another drive.

I will try bleeding another time or two, however currently not getting any bubbles and peddle feels normal. Thanks

[DRACO A5OG]

"rebuilt" who did the rebuild and what was done?

10 years, SS lines also deteriorate internally over time. I am changing mine back to OEM rubber because rubber will show signs when they are ready to go, SS not so much.

[meichten]

PMB rebuilt the calipers

[jpnovak]

rubber flex lines need to be replaced. They are likely swollen internally and preventing fluid from returning to the MC.

Make sure to do all 4. When you bleed you are releasing the pressure and the caliper piston can retract properly.

[914/6]

pedal free play at master cylinder..none holds pressure

[DRACO A5OG]

Originally Posted by meichten

PMB rebuilt the calipers

They do good work from members here but did you check the piston position was 20 degrees? I ask because my buddy just got his and two of them were out of spec.

I would check them.

To test the brake line theory, disconnect and watch the flow as helper presses the pedal.

[Amvcane]

Originally Posted by DRACO A5OG

To test the brake line theory, disconnect and watch the flow as helper presses the pedal.

I’m having the same symptoms on my left rear caliper just after I rebuilt them. Had the problem before the rebuild and thought it was a stuck piston.
Before I replace the lines I would like to make sure that this is the source of the problem. Could you please explain what I’m looking for during this test? Thanks.

[meichten]

Jamie, lines are currently all steel hard lines to flex steel to calipers.

I’ll give the calipers another look over Draco.

Is there anything else in the pedal itself that would put more back pressure on the MC after release? The only think I see in the diagrams are the linkage to move the pedal in/out in the cluster.

[jpnovak]

Are your pedal bushings sticking. If the pedal does not return all the way the MC will not release pressure. Also check the linkage rod between pedal cluster and brake booster for binding. They can be adjusted too tight and prevent return.

[DRACO A5OG]

Sorry re-read your post and the other thread. I did not realize all four were binding, yeah it might be mechanical, there should be a little play before brake activate at the pedal. Maybe PO took it all out?

Try this, put her up on all fours, and gravity bleed ( open the valves ) all four at the same time and make sure reservoir does not run dry. Have helper keep filling as you observe the flow.

[meichten]

Thanks all, will get to these additional steps next weekend.

[theenico]

Originally Posted by 914/6

pedal free play at master cylinder..none holds pressure

I agree.

Meichten: When you said you replaced the MC, that was my first thought. Check all linkage adjustments from the pedal to the MC. If it's even a little off the MC piston can cover the vent/compensating port in the MC. If those ports are covered, there is nowhere for the fluid to go as it heats up and expands. The result is what feels like all four calipers sticking. What's actually happening is your brakes are slowly applying themselves.

[meichten]

After some additional tests and reading, believe the issue I’m having is due to the linkage between the new MC and existing booster. Everything works fine until the brakes heat up, then the calipers start to stick. Meaning the pushrod is still extended too far into the MC even with no pressure on the brake pedal. So ? Is does anyone know if/how to adjust the pushrod length on the booster pictured below? Thanks

[frankc]

Originally Posted by meichten

So ? Is does anyone know if/how to adjust the pushrod length on the booster pictured below? Thanks

I had this same issue a few years back after rebuilding the pedal cluster. For power brake cars, the pushrod is adjusted in the pedal cluster area at the linkage rod that goes from the brake pedal up to the brake booster pivot. Let me know if you need a picture, but it should be obvious once you remove the cover over the cluster.

[meichten]

A picture would be great, did you shorten the brake pedal linkage?

[meichten]

Adjusted the linkage and still have the same result. I do not see a way to adjust the push rod length in the booster any ideas?


Does anyone know the impact of the brace in the picture below? It was unattached when I purchased.

[Jeff Higgins]

Originally Posted by meichten

Jamie, lines are currently all steel hard lines to flex steel to calipers.

Your "flex steel to calipers" are, I assume, braided stainless steel, and we're just having a bit of a terminology issue.

Even braided stainless steel flex lines are rubber on the inside. That is what DRACO A50G is getting at here:

Originally Posted by DRACO A5OG

"rebuilt" who did the rebuild and what was done?

10 years, SS lines also deteriorate internally over time. I am changing mine back to OEM rubber because rubber will show signs when they are ready to go, SS not so much.

The outer braided stainless steel sheath is only there to keep the rubber hose from expanding, thereby providing a firmer pedal. They will deteriorate internally in exactly the same way plain rubber brake lines will. Like DRACO A50G is saying, the stainless sheathed lines will fool you by not deteriorating visibly on the outside. I've seen plenty of these well and truly clogged up from deterioration that still look great on the outside.

[meichten]

Jeff, thank you and thank you DRACO A50G, correct they are the braided steel. Believe they are about 15 years old.

[DRACO A5OG]

Let us know if swap fixes your issue.

[meichten]

Did some more diagnostic testing, confirmed booster hose and check valve were good, booster seemed faulty. Picked up extra booster from ACAR124 (local), swapped out the booster and waalah brakes working as expected now, not seizing after they warm up!

Replacement lines are on order, will swap those as well for good measure as the current ones are 15 yrs old.
Thanks All for the help, glad to be back safely on the road.