Disassembling Caliper Pistons
 

Hello,

Are caliper pistons serviceable? I have the alloy "S" calipers and have the stainless steel pistons. These pistons look fine, but I noticed they have an internal "hub" that is secured with a snap-ring. I thought it would be prudent to take them apart to ensure thorough cleaning if this is possible.

Any thoughts?


JA

I believe that is for the "disk balancing", I would think a good soak in brake cleaner and some air through it would be enough. I just found that me drivers side calipers have that "feature".

Not sure I've heard of "disc balancing". Is this something that is done inside the pistons?

Any other experience disassembling these SS caliper pistons?


Thanks,


JA

The internal mechanism [under the snap ring] of all early 911 caliper pistons is for compensation of excessive deflection due to too loose wheel bearing adjustment, etc. The 'hollow' fits over the stud/post in the middle of the caliper bores ... and serves to resist movement of the pistons back into the bore. Sort of a mechanical one-way 'valve' ... free to move outwards, while resisting the forces tending to push the piston back in the bore. You can, of course, slowly push the piston in with a C-clamp, or 'C-clamp style' Vise-Grips.

Yes, you can take the snap ring out and remove the guts if rust is evident ... just be sure to take a picture or draw a diagram of the sequence of the stacked parts as you lay them out on a paper towel!!! Naval Jelly is good for cleaning the rust off ... silicone grease should be applied to everything before reassembling the 'stack' under the snap ring!

None of the above is documented in factory publications ... I believe it is mentioned in the Clymer <b><i>Porsche 912 Handbook!!!</b></i>

My observation is that the clamp and spring under the snap ring draws the pad back off the rotor a minute amount between use of the brakes. Just like self adjusting drum brakes, the pad advances along the stud during its life. As far as centering, I have heard (only heard, not seen) that racers leave out the clamp and spring. I don't know if the stud would do much if that were the case.

There are more pistons out there than those little clamp gizmos because the new replacement pistons don't come equiped with them. You have to reuse your old ones. That might be why some run w/o them. They have no choice.

Like Warren says, don't drop the parts in the lawn......

Warren and Milt... Thanks!

Incredible information here... I'll take it apart and keep close track of everything, clean it and grease it. I didn't see any rust at all from the outside, but I'll check again once I get them apart.

Thanks again!


JA

Would you mind posting the order of the stack of parts? I took mine apart several years ago, and while they are back together, I was never certain if I had it right. Thanks.

Update
 

Hello,

Well, I finally got around to disassembling these caliper pistons. Kind of anti-climatic, really. As far as I can tell, there are only three pieces inside the piston... the snap ring, the sprung "hub", and the little brass bushing.

Oh well, for those interested, here are a few pics in the order of disassembly.

http://forums.pelicanparts.com/uploa...1104117097.jpg

http://forums.pelicanparts.com/uploa...1104117141.jpg

Oh, and one more question for Warren...

I will definitely apply some silicone grease when reassembling, but I just wanted to inquire about the compatibility of silicone grease and brake fluid...ok apparently? Also, I have Sil-Glide from Napa.. is that satisfactory to use?


Thanks,

JA

John,

The fact that Ate used to supply a tiny 1 cc blue tube of unlabeled silicone grease [looking and feeling just like Dow Corning 4 or 4X] in their master cylinder rebuild kits told me that it was OK! I also recall that VW officially recommended an unobtainable product called Ate Brake Paste in the early '70s ... to be used on all cups and seals during a rebuild. I have always used Dow Corning 4 or 4X when available at hand , and never had a problem on rebuilt wheel cylinders, master clinders, or calipers using the Dow Corning greases.

I have used Syl-Glyde in a pinch, but recently noticed that the contents of a 10-year old tube had turned dark yellow-brown ... something that had never happened to my 40-year old container of military surplus Dow Corning 4 I have had since 1968!