Brake caliper problem

[johnb]

I posted a long thread about this a while back in rennlist, and never got to the bottom of the issue. Now I'm about to tackle it again.

Years ago I replaced all the seals in my brake calipers (with genuine Brembo seals sourced from Paragon). The pistons all looked fine and I didn't replace any of them. But afterwards, the pedal was never consistent. It would be a little low, then a few pumps would get it high, then it would be low again. The braking power was there, but the pedal feel was just annoying.

So earlier this year I decided to get to the bottom of this. Now I know what you're thinking - there must have been air in the system. Well I went with that assumption, and tried everything I could - and sometimes I have gotten air out when I bleed them, but it never made any improvement to the consistency of the pedal. It still sometimes goes lower than I'd like. At this point I'm pretty sure that air is not the issue. In this same timeframe, I've also replaced the wheel bearings, so I know it's not that.

To cut a long story short, by isolating various parts of the system, I've narrowed it down to the rear calipers (both), and it seems to be that the pistons are just being retracted too far by the seals. This seems to be why I can get a really high, solid pedal with a few pumps, but next time the pedal is low again. So I concluded that there must have been something wrong with the seals, and I bought new ones (again definitely genuine Brembo). When reassembling the calipers, I had a tough time getting some of the pistons in. I made sure the bores and grooves looked clean. I scrubbed the grooves out with a toothbrush and some solvent, and used proper brake caliper assembly lube for reassembly instead of just brake fluid. But some pistons just took a lot of force to go in anyway. Likewise getting them out using compressed air took a lot of pressure - they seemed to get stuck and then pop out very violently.

And afterwards, the problem is still there. I put in a set of worn down pads and spent ages pumping the pedal to get the pistons out, and then pushing them back in again in an attempt to "exercise" the seals, but that hasn't helped either.

Has anyone ever seen anything like this? I'm ready to just write these rear calipers off as "haunted" and find another pair somewhere, but I just wanted to see what everyone things first. I'm out of ideas.

[Tom]

I rebuilt my calipers a long, long time ago. I can't quite remember why, but seem to recall one was leaking. The pistons and seals were super hard to source. Porsche didn't have them, and I found them from some obscure source I'd never be able to find again. I also remember that Porsche changed the exact piston sizes a lot, and the only way to be sure was to mic the originals.... A couple of guys I trust told me not to mix new seals and old pistons or they may not seal well, so if I had to guess that may be why yours are acting wonky. Just a guess. But if you can't find a piston/seal set that fits, I'd probably just find a good used set...

[Darwin]

About 10 years ago I had all four of mine rebuilt by a place in Seattle. Name of the place was Goldline brakes. He sourced all his own replacement parts, and did a fantastic job. He mentioned that you had to be careful when replacing everything, but didn’t go into detail.

[Thom]

Are you sure you tightened the wheel bearings enough? I had a similar issue recently when upgrading to big blacks and the front wheels bearings were too loose.

[johnb]

Are you sure you tightened the wheel bearings enough? I had a similar issue recently when upgrading to big blacks and the front wheels bearings were too loose.

Yeah, good point. But in my case the issue is definitely with the rears, and the rear wheel bearings are not adjustable. I've replaced mine and they're rock solid. I've also checked runout on the discs with a dial indicator.

[Thom]

The issue in my case never quite disappeared, even after tightening the front wheel bearings. The braking power is absurdly good, but only after pumping the pedal a first time. I think this was talked about on the 'other' board several times and the issue may be the master cylinder.

[johnb]

So I have another set of calipers and I was hoping to remove the slide plates to clean them up. That would mean destroying some of the screws since they're stuck in there pretty badly.

Unfortunately I'm not sure if I can replace them now. I got one of them out, and it turns out they're weird screws with a tapered unthreaded part:

20210926_202408.jpg (2.65 MiB) Viewed 2519 times

Can you buy these anywhere? I compared a 6x1mm socket head screw, and it really does't fit the same. The holes in the plate are slightly wider, just right for the tapered screw. The ordinary ones leave a tiny bit of play. It might not be a big deal once it's all tightened up - but then, why are they like this at all? Does it matter that much? Would I get away with ordinary screws?

[Darwin]

Is this what you’re looking for?

https://www.paragon-products.com/Calipe ... 959.01.htm

[johnb]

Is this what you’re looking for?

https://www.paragon-products.com/Calipe ... 959.01.htm

The screws, yes, but not the plates.

[Darwin]

If you’ve already gone through the trouble of removing them, why not replace them?