Brake caliper problem

[johnb]

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

I haven't removed them yet. I don't want to replace them because there's nothing wrong with them and they're $50 each which would be $200 to do both calipers, when all I need is the screws.

[Tom]

I'd guess that's more of a 'part' than a screw you'll find in the wild. If you don't want to spring for the kit (no pun intended), then maybe @lart951 could supply some. On the other hand, if you got one out, maybe you'll get equally lucky on the others....? Did you look in PET to see if the screws have their own part number?

[Tom]

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

I haven't removed them yet. I don't want to replace them because there's nothing wrong with them and they're $50 each which would be $200 to do both calipers, when all I need is the screws.

Leave them in place and use brake cleaner and a toothbrush??

[Darwin]

I checked PET. Unfortunately they only list the plate and screw as a kit, with no specs on the screw listed. If you want the actual fastener, your only option may be to buy the kit. Maybe someone else will chime in with specs or a source?

[Thom]

The last time I tried to rebuild front calipers we had to weld to each screw a piece of metal to have enough force to loosen them, and we could not find any other method to get them out. This method, certainly used by many people on other cars using fixed calipers, potentially damages the plates and I believe this is why the rebuild kits includes both a new plate and 2 screws. With new plates we can be confident that there will be no more fitment issues with new pads... I do not think I am the only one who had to slightly machine down at least once in their 944 ownership the edges of new pads which could not slide freely in used calipers with slightly bent plates.
Replacing all these is for sure a budget, but brakes are I think a safety topic that I do not want to mess around with.

[johnb]

Well thanks for the input everyone. The plates are in really good condition and I will probably just clean them where they are. It'll take a bit more than a toothbrush, which is why I wanted to remove them, but since I don't even know for sure if these calipers will fix my problem, I'm not doing anything else to them until I have at least got them on the car and tested them.

They look promising though - the pistons were a little stiff when I got them, but after some excercising they seem to be moving with more precision than my old ones. I'm just waiting for new dust boots to arrive and then I'll put them on.

[Darwin]

Just be sure to make sure they aren’t bent. Mine were turned up on the inside and prevented installation of new pads.

[dr bob]

FWIW, and since we share brakes --

I accidentally pushed some air into the system during a fluid flush/bleed exercise. I ended up with air trapped in the ABS unit at a high spot, and no amount of conventional or power bleeding would move it to somewhere it could be bled out. Final solution was connecting a vacuum pump to the lines at the MC ends one at a time, pulling hard vacuum through a little liquid trap, then feeding new fluid in from the caliper ends of the plumbing, through sections of vacuum hose slid over the steel plumbing. (Hemostats as clamps...) Pedal symptoms were the same as you describe to start with.

Bleeding the calipers completely is a multi-step process that bleeds the inside chambers first, then the outside, then both again the same way. It can help if the caliper is unbolted so you can rotate it after the first inside-chamber purge, get the crosspipe to the top, tilt, then bleed the outer, rotating the bleed screw to top part way through.

On the 928 anyway, there was a change in the seals and dust boots early in 1989. Did your new seals and dust boots match the ones that came out?

Some factory notes say that the systems were vacuum-filled like this when the cars were assembled. I'm not sure how they pulled vacuum on the back of the master cylinder without sucking air into the lines.

[johnb]

FWIW, and since we share brakes --

I accidentally pushed some air into the system during a fluid flush/bleed exercise. I ended up with air trapped in the ABS unit at a high spot, and no amount of conventional or power bleeding would move it to somewhere it could be bled out. Final solution was connecting a vacuum pump to the lines at the MC ends one at a time, pulling hard vacuum through a little liquid trap, then feeding new fluid in from the caliper ends of the plumbing, through sections of vacuum hose slid over the steel plumbing. (Hemostats as clamps...) Pedal symptoms were the same as you describe to start with.

Bleeding the calipers completely is a multi-step process that bleeds the inside chambers first, then the outside, then both again the same way. It can help if the caliper is unbolted so you can rotate it after the first inside-chamber purge, get the crosspipe to the top, tilt, then bleed the outer, rotating the bleed screw to top part way through.

On the 928 anyway, there was a change in the seals and dust boots early in 1989. Did your new seals and dust boots match the ones that came out?

Some factory notes say that the systems were vacuum-filled like this when the cars were assembled. I'm not sure how they pulled vacuum on the back of the master cylinder without sucking air into the lines.

The dust boots were definitely the same. The seals, it's harder to say. It's years now since I did the original rebuild, but recently when I replaced the seals again, the ones I removed seemed to grip the piston a bit too tightly (testing the fit out of the caliper). The new ones were just a tiny bit looser. But ultimately the pistons didn't go in any easier with the new seals.

I have another set of calipers now that seem to be in OK shape, but I'm having a little trouble with some of the slide plates so I need to get that figure out before I can test them.

I did consider air being trapped in the ABS unit, and I even started work on designing a device to cycle the ABS valves and pump. But then I ruled the ABS out by blocking off the circuit after the ABS. The pedal feel is fixed when I do that, so that should rule it out. Because of that I never got around to finishing the ABS cycler, but I still plan to finish it since it might be useful.

[dr bob]

Many owners have said that the pedal gets firmer when they installed new brake hoses. The Goodridge hoses are popular and reasonably priced. Installing them takes a little planning and care to make sure that sections of hard pipe don't drain out as you fiddle with the connections. I try and do them one-by-one, and use a short section of vacuum hose with a hemostat clamp as a cap on the steel line. I later found some bulk bleeder caps that do an OK job.


On the little slide plates and screws --

It seems common that corrosion behind the plates is causing them to bend up tighter on the pads. The screws are installed with red Loctite, meaning there needs to be some heat to soften that before the screw will come out. Of course we have those big aluminum caliper bodies that carry heat so nicely. What we'd really like, counterintuitively, is to heat the screw quickly to spit-hot, and use an impact driver to get the screw out before it cools much. When the big aluminum chunk heats up it tends to shrink the hole a bit, while the stainless screw doesn't expand a lot with the few hundred target degrees (Fahrenheit). A good propane or even a MAPP gas torch is plenty. It's certainly OK to heat the whole thing (dedicated garage toaster oven...) to 250-300º or so and not risk damaging the seals or other rubber parts, looking to soften the Loctite, while a differential between the screw and the aluminum caliper body can help break a corrosion bond. I've done it both ways with relatively equal success, but I've always lived in dry desert climates. Either way, use the impact driver on the screw. Once that's out you'll see the buildup of brake dust and corrosion that's been plaguing the back of the plate.