930 Front Caliper Rebuild Question About Scraper Rings
 

About 2 years ago the front passenger caliper was hanging up so I put in new seals and scraper rings. I did not separate the caliper halves but did run the pistons in the bores w/o seals or scrapers and the pistons freely moved. Visual inspection of the bores showed no crud or other obstructions. They were quite clean. The caliper worked quite well after the re-seal.

Fast forward to today and I decided to replace the rotors. With the caliper off, I couldn't push the pistons back in. I used a piece of hardwood with the caliper on the bench and with all my weight, could only press in 2 of the pistons.

So I popped out the pistons to inspect and noticed the scraper rings seem to have stretched. Here's some pictures of the scraper rings sitting on the bench and the gap between the pistons and the rings can be seen. They were not like that when I installed them.

What is going on here? The original scrapers I removed were not stretched.

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It looks like the rubber swelled for some reason. If the rings came in contact with petroleum based grease or some other petroleum product, that could explain it. This is the reason we only use silicone based grease on brake systems. Obviously they need replacement.
Rob

I did us Permatex Ceramic Extreme Brake Parts Lubricant item # 24125 when assembling the calipers. Was this not the right lube? I got the info from some folks well versed in caliper restoration.

If this isn't the right lube, what is?

BTW, thanks for the feedback.

Rebuilt my 930 calipers this winter and simply used brake fluid when reassembling.

Not the right stuff! For metal-to-metal contact only. Not for use with the seals or scrapers.
What I use for assembly of pistons and seals that may be wetted with brake fluid is ATE Brake Cylinder Paste; I got mine 35 years ago as a VW part now discontinued:
https://www.coolairvw.co.uk/Item/Shop_by_Vehicle~Beetle_Products~Beetle_Brake_Produ cts~Brake_Pads~Copper_Grease_-P-_Brake_Cleaner/AC6001298/ATE_Brake_Cylinder_Paste_-OB-180Gr_Tube-CB-.html

contact brembo about what to put on the seals.
I soaked mine in it for 24hrs.

Thanks. Kinda came to that same conclusion after examining the scraper rings. I thought I was told by a well-known caliper restorer to use this. I was wrong and don't want any doubt placed on any of the caliper restorers. My bad.

I will say this that pulling the caliper off and popping out the pistons is not that difficult. Replacing the scrapers is quite easy and not much expense. The seals on the other hand are not that easy or inexpensive to swap.

I have some of the Porsche caliper lube on order. For right now with new scrapers, I've used Sil-glid as an assembly lube. Maybe just as bad but the pistons did go in quite smoothly.

Not sure why this passenger side caliper is dragging. The rotor always 5-10C higher in temps than the driver side. Both have new seals and scrapers. Pistons and bores were very clean the first time I rebuilt them using just brake fluid during assembly.

Could be that the flexible hose to the passenger side is getting plugged from the inside and pressure isn't being released after letting off the brake. Were the flex hoses replaced when you re-did the calipers? If not, they may look good from the outside, but they swell on the inside causing fluid flow to be restricted.

I did replace the flexible hoses. The originals were in good shape. Thanks for the input.

The previously swelled scrapers might be the culprit. My understanding is the groove for the scaper is slightly conical on the pad side, and the scraper retracts the piston lightly after pressure is released. Swelled scrapers would negate this.

To clarify, the cross section of the scraper becomes a parallelogram when under pressure and the piston moves towards the rotor. When pressure is released the cross section of the seal goes back to square and retracts the piston slightly.

I have tried to read this carefully, but keep thinking the terms 'scraper ring' and 'seals' are being reversed.

I rebuilt the calipers for my 930 project. when I checked a complete kit was crazy expensive. turns out those outer rings are the scraper ring and were $40+ each. learning whatever the cost of a part for a 911, 2-3X for it if a 930. mine seemed fine so I only replace the inner rubber seals which were not expensive.

I learned about these before working on the 930 calipers when rebuilding the calipers on my 951 DE car. they also have the outer steel/rubber scraper rings.

my past experience rebuilding calipers were from 911s and other cars that used outer 'dust boots'. so when I pulled the calipers off the 951 my initial suspicion was the rubber dust boot portion had been destroyed? after removing the scraper rings from one caliper realized they were different than anything I had seen before.

call the shop for parts and glad I did. found that one of the scarper ring sizes is NLA. so ended up just replacing the inner rubber seal on the other calipers and had to fab a dust boot for the pistons where I had removed the NLA scraper rings. bent up the metal so could not reuse.