mx22

Hi guys,

Got a couple of questions as I'm preparing to do "rebuild" calipers. I found this video on YouTube:

http://www.youtube.com/watch?v=HCLqUREvKm8

It's 944 Turbo S caliper, but I think S4 brakes are the same or very similar.

Guy is replacing pressure seal, but I don't see him either removing or putting in dust boots. Do I need them and if so how do they go on a piston?

He also mentions that he had trouble putting back smaller diameter piston and had to use special tool. Do I really need it or any other special tool to accomplish the job? I was thinking of simply using clamps...

Using air compressor to remove pistons - I have one back from when I used to paint model planes. Not sure if I will be able to use it or even if it will be powerful enough. Any other safe ways to remove pistons?

Thank You!

Rob Edwards

Alexei-

Before you do too much disassembly, if you have the original 1987 calipers on your car, make sure that you can actually get the seals you need to do the R&R. I recall that people cannot get the parts for these 'early' S4 calipers, maybe that situation has changed.

Recent thread with a factory TSB describing early vs. late calipers:

https://rennlist.com/forums/928-foru...ifference.html


EDIT:

As far as pushing pistons out with compressed air, here's how I did it:

https://rennlist.com/forums/928-foru...not-to-do.html

mx22

Rob,

I've been annoying Roger for the last couple of weeks with my spare set of calipers. For seals, he pointed me to this page:

http://www.zeckhausen.com/stoptech/c...es.htm#rebuild

Fronts need 4x 36mm and 4x 42mm, rears 4x 28mm and 4x 30mm (if anyone will be buying these from the link above, keep in mind that they sell them in pairs - i.e. you only need 2 kits of each to do all 4 calipers).

I think I'm all set with parts I need for the rebuild, just hoping I won't damage pistons in the process.

Looked at your post, very informative - thanks a lot! I still can't wrap my mind around what you did to get pistons out. Somehow I doubt I can connect that beast (even if I can replicate it) to my air compressor...

Rob Edwards

Alexei-

I would be happy to toss the Rube Goldberg air gun/pressure fittings tool in the mail to you if you like, just PM me an address. You'll still need access to compressed air but any corner garage might let you 'rent' $5 worth of air to get the job done..


Since that post I rebuilt my GTS's calipers, and had better success just sticking a length of 2x4 thru the middle of the caliper, so that each piston could be pushed out of its bore a little bit but then stop against the wood. You don't want any one piston to pop out of its bore, so that slowly all four pistons push out evenly. This ended up being a lot easier than all the machinations with wood blocks that I posted in the above thread. Once they're sticking out, some could be wiggled out with fingers, and the others could be grabbed with channellock pliers whose teeth were wrapped with 2-3 layers of duct tape. But then I wasn't trying to reuse pistons, so I could be cavalier about using pliers. Goes without saying, be really careful with the pistons and their bores......

mx22

Rob,

Thanks for the offer, but I'm going to be doing the job either tomorrow or over the long weekend (actually I thought to tackle it later on today, if I can get the pistons out), too late to ship anything to me - I'll have to figure something out locally. And thanks for the tip with 2x4 - sounds easier then using clamps and removing pistons one by one as in that video above.

mx22

Looks like I can use my old hobby compressor, just need to buy new hose. Would 30psi be enough to get calipers out? Don't really want to spend $100+ on a compressor I will use once...

Gary Knox

The pressure required to remove the pistons in a set of calipers that haven't been serviced in 15-25 years can be pretty significant. I've developed as system that works well for me on our four piston calipers (F or R).

Remove the calipers from the car and drain the brake fluid from the calipers. Remove the crossover tube between the two caliper halves. Cut a piece of wood that is the "length" of the brake pad, and about half the thickness of the opening between the two caliper halves (this will be inserted between the halves and acts as a "catcher" when the piston blows out to keep it from being damaged). On the outside half of the caliper, close the bleed valve, and use an air "blower" that has a rubber tip on it and a trigger to actuate airflow. Press the rubber tip up against the opening where the crossover tube connected in that caliper half and press the trigger. (I start with about 75 psi of air). IF that doesn't move one or both of the pistons, keep increasing the air pressure until ONE of the pistons pops out. Then remove the dust boot from that piston.

I then clean that piston and it's cylinder thoroughly with lacquer thinner on a rag (or brake cleaner), put a thin coat of Porsche caliper piston grease (expensive!!) on the piston and also the cylinder. I insert that piston back into it's cylinder, then get another small piece of wood that fits between the one already in the caliper and THAT piston that I just inserted, to keep it from being blown out again. Now, apply the air pressure again in the same manner to remove the 2nd piston from that side (may need higher air pressure). Repeat the cleaning/lubing work and re-insert that piston. Of course, if you are using new seals, remove the old seals and install the new ones before you put the pistons back in place.

Now, work on the "inner" part of the caliper. This time I put bleed screws in both ends of that caliper, and apply the air pressure through the opening where the brake hose was connected. Also, Repeat the exact same process on this half of the caliper.

IF the old dust boots are in good shape (no cracks, holes, tears, etc.) and I want to re-use them, I put them in a jar with enough brake cleaner to cover them and let them soak for a half hour or so. Then, remove them, wipe carefully with a paper towel, and let them "dry".

I also "exercise" the pistons several times until they will come out easily with about 35 psi of air pressure, and can be easily pushed back in the cylinders with finger pressure. Then, with the piston emerging about 1/4" from it's cylinder, I install the new or cleaned dust boots, push all the pistons back in place and they are ready to either re-install, or paint (I always use the expensive, but good epoxy finish, as it is so much more durable than the inexpensive spray paints). Put the crossover tubes back in place, and you are done with the rebuilding process!

When all of this is done, installed on the car, and the wheels re-installed, I find the wheels rotate MUCH more easily than they did before, the brake dust build up when driving is significantly reduced, and even the fuel mileage seems to improve a bit!!


Gary Knox

mx22

Gary, thanks a lot for the insight. Would this air compressor be sufficient?

http://www.sears.com/shc/s/p_10153_1...&blockType=G15

I want something small and not too noisy...

Thank You!

Gary Knox

Alexei,

Any compressor that puts out up to ~125 psi should be sufficient. I think I've had to go to 110-115 a couple of times to get a piston out, using the method described above. Frequently they will pop out (and "POP" is the right term!) at 90-100 psi, but not always.

Good luck, this is a maintenance job well worth doing.

Gary-

bimmen325

Just so you know, I have a set of front calipers for sale, zero miles on them.

mx22

Rebuilt the calipers yesterday. I thought it would be a 2 hour job, but it stretched for almost 5 hours.... My car is an 87 and it turned out that caliper pistons are different from Gary's write up above - from what I understand all '87 and early 88 cars come with an early style of the S4 brake. There is no dust boot... I also ended up reusing the original sealer rings as they were in great condition.