Brake Caliper Issue?

[dr bob]

On the subject calipers, there are some spring shims under the pins that hold the pads in. The caliper pistons are normally fitted with some pads that clip into the piston bore, and adhere to the backs of the pads. Both of these help with rattle and squeal, plus the ones that clip into the pistons help retract the pad with the piston when pedal pressure is removed.

In the caliper recesses where the pads slide in, there are spring plates on the 'upstream' end. They are held in by small stainless button-head socket capscrews. The spring plates slightly load the pads forward in the recesses, and eliminate clicking you might otherwise notice when applying the brakes. That also protects the pads that clip into the pistons, so they don't pull sideways on the pistons in their bores.

The challenge is that brake dust and moisture conspire to create concrete under those spring plates. With dissimilar metals, there's some corrosion possible there too. A typical pad swap doesn't include R&R of the screws and plates while the pads are out, but knowing what we do about the situation it's easy to add. For cars that have been driven a lot especially with metallic pad compounds, the crud and corrosion under the spring plates can get so thick and dense that space for the pads is reduced. Pads no longer retract since they are clamped in the caliper frame.

Remedial work includes getting the screws out as the major challenge. The screws normally get a drop of thread locker to keep them in place, so the removal process starts with some heat to soften that. Just enough to soften the thread locker, but not so much that the aluminum grows into the threaded hole. Then an "impact driver" is used to remove the screws. It is important to clean all the crud out of the drive opening on those screws before you try and removed them, or risk the soft stainless getting smeared by the driver before the screw comes loose. The little spring plates come out, and you have a chance to clean the brake dust and corrosion 'cake' out, and follow the thread in each hole with a chaser or bottoming tap (second choice) to make sure it's clear. Then reinstall with new screws and a tiny bit of thread locker high on the screws. Were I shopping for them today, I might look for some Torx drive replacement screws instead of the hex.

[NCGermerican]

I used to get a rattle on my rear brakes when going over bumps. Turned out my rear pads were missing the shims. I put those in and the sound is gone. I don't know if yours requires the shims or not though.... Just a shot in the dark.

I appreciate the help! However, I installed brand new pads (Hawk Ceramic) that had the shims already installed on them. No change in the noise.

[NCGermerican]

On the subject calipers, there are some spring shims under the pins that hold the pads in. The caliper pistons are normally fitted with some pads that clip into the piston bore, and adhere to the backs of the pads. Both of these help with rattle and squeal, plus the ones that clip into the pistons help retract the pad with the piston when pedal pressure is removed.

In the caliper recesses where the pads slide in, there are spring plates on the 'upstream' end. They are held in by small stainless button-head socket capscrews. The spring plates slightly load the pads forward in the recesses, and eliminate clicking you might otherwise notice when applying the brakes. That also protects the pads that clip into the pistons, so they don't pull sideways on the pistons in their bores.

The challenge is that brake dust and moisture conspire to create concrete under those spring plates. With dissimilar metals, there's some corrosion possible there too. A typical pad swap doesn't include R&R of the screws and plates while the pads are out, but knowing what we do about the situation it's easy to add. For cars that have been driven a lot especially with metallic pad compounds, the crud and corrosion under the spring plates can get so thick and dense that space for the pads is reduced. Pads no longer retract since they are clamped in the caliper frame.

Remedial work includes getting the screws out as the major challenge. The screws normally get a drop of thread locker to keep them in place, so the removal process starts with some heat to soften that. Just enough to soften the thread locker, but not so much that the aluminum grows into the threaded hole. Then an "impact driver" is used to remove the screws. It is important to clean all the crud out of the drive opening on those screws before you try and removed them, or risk the soft stainless getting smeared by the driver before the screw comes loose. The little spring plates come out, and you have a chance to clean the brake dust and corrosion 'cake' out, and follow the thread in each hole with a chaser or bottoming tap (second choice) to make sure it's clear. Then reinstall with new screws and a tiny bit of thread locker high on the screws. Were I shopping for them today, I might look for some Torx drive replacement screws instead of the hex.

I've touched on that. There is also a much riskier solution, especially if those screws won't come out (after 170K miles) and that's to grind off (VERY carefully) some of the ends of the brake pads. I did this as the "quicker" solution, but only slightly helped with the noise/feel. I'm going to be swapping out the master cylinder, reservoir and if I get get them off, the old stanless lines for a new set I purchased from 944online. If none of that solves the issue, then it HAS to be those spring plates. Unfortunately, I refinished the brake calipers and they look gorgeous so I have to be very careful with the methods of removing those spring plate screws.

[icb]

You guys are making me really happy that I haven't succumbed to the temptation to put the 4 piston brakes on my 944S!!

[NCGermerican]

You guys are making me really happy that I haven't succumbed to the temptation to put the 4 piston brakes on my 944S!!

Hey Ian! I did that on my 87 S, and it actually added to braking performance over the stock brake setup.

It wasn't really that bad of a job - just need the calipers, spindles/hubs from an 87+ turbo (non M030) and a turbo bias valve. But yes - definitely more that could go wrong....

[Zirconocene]

I have a post over on Rennlist from when I refurbished the brakes on my 928 S4. Removing the screws was a challenge and I had to go the route of having a friend weld a nut to the top of the screw. Once that was done (use as large a nut as you can get away with, fyi) it was very easy to turn out the screws. All the heat from the welding also played a role.

The spring plates are stainless steel and should clean up very nicely. Underneath them will likely look like a superfund site; lots of corrosion and grossness.

Finally, the screws are easy to source; McMaster has some nice options available that will fit and not stand any higher than the OEM parts.

Cheers

[NCGermerican]

Finally, the screws are easy to source; McMaster has some nice options available that will fit and not stand any higher than the OEM parts.

Cheers

Do you happen to know the size and thread pitch of that screw? According to PET, the screw is bundled with the spring plate (at $75 per kit!) so the specifics of the screws are not listed.

951 352 919 01 Spring Plate Kit (Top)
993 352 959 01 Spring Plate Kit (Bottom)

[Zirconocene]

I'll get you those measurements tonight; my online history at McMaster doesn't quite go back far enough so I wasn't able to look it up just now.

Here's the thread I was referencing, where I also did not disclose the McMaster part number or screw specs, d'oh!: https://rennlist.com/forums/928-forum/1 ... fresh.html

Cheers

[dr bob]

Remind me if those spring plate screws are stainless? If not or depending on the grade, an induction bolt heater might be a good option for getting the old ones out, melting the threadlocker without swelling the holes in the aluminum closed.

[NCGermerican]

Remind me if those spring plate screws are stainless? If not or depending on the grade, an induction bolt heater might be a good option for getting the old ones out, melting the threadlocker without swelling the holes in the aluminum closed.

I'm not sure what the screws are made of. I would think Porsche (or....Brembo) would think stainless would be too soft to use an allen head with all of those head cycles AND thread locker. I could be wrong though.