eclou

What a PITA!!!!

I rebuilt my 2 rear calipers using new seals from Paragon. Used compressed air to pop out the pistons. I managed to nick a couple when they popped out (they shoot out!) and polished them smooth with 1200 grit. After cleaning the caliper bores - they looked and felt good - I removed the old seals and put in fresh ones. The old ones looked okay.

Putting back in the pistons was the hard part. I oiled them with brake fluid. Only about half went in smoothly, the others required nudging with a C-clamp. One was so tight that the c-clamp broke. This was a bad sign that I chose to ignore.

When I re-installed and pressurized with the Motive brake bleeder, a pool of fluid ran out of my driver's side caliper. Upon dissassembly I found that I managed to shear one of the seals with the piston. Replaced it again and no more leaks.

Today driving was an exercise in disaster. 15 minutes and my brakes started to go away. I thought it was just air in the system but then the "!" light came on. Back in the garage the pass wheel was coated in brake fluid. Disassembled that caliper and there were 2 more sheared seals. Replaced those two, re-bled, and now everything is fine. Going to the track on Thursday.


Pointer/lesson learned - the pistons should slide back in relatively easily. If they do not then you WILL tear or shear the seal. The key is to polish the chamfer on the inboard side of the piston (the side that slides into the bore). My pistons were totally clean on the sides, but the seals that I had problems with has just some tiny rough spots/crud on the chamfers that would hang-up on the seals and cause a shear.

jimbo1111

You shouldn't have used break fluid as a lube. It swells the seals.

Sam Lin

Very good post - good condition calipers like just after a rebuild should only require finger pressure to press the pistons in - my last set it was just thumb and 2 fingers squeezing them in. A big reason I never recommend a C-clamp for pushing pistons back - there's no feel for the amount of force you're applying, and it's very easy to **** a piston with a small C-clamp head and then gouge into it as you try to tighten more. If you need a clamp, use a Quick-Grip like one of these:

http://www.irwin.com/irwin/consumer/...5&search=false

They limit the amount of force you can apply, and the rubber jaw helps spread the load over the entire piston.

Also, for future reference, when popping pistons out with compressed air, put a block of wood in the middle of the caliper that's thicker than the rotor, and inject compressed air very slowly - you'll see the piston slowly move out. On calipers in decent shape, all 4 will come out together and clamp against the wood. If not, you need to put a clamp against the one that came out, and slowly bring another one out - repeat until all 4 are hanging halfway out. Then, don't use the air! Just grab them by hand and pull them out. It'll prevent you nicking the pistons.

Last - you mention some bits of crud on the seals that gave you issues - you didn't clean the parts enough before reassembly. If you wouldn't be comfortable licking any part of the piston, seal, or bore, it's not clean enough.

Sam

eclou

On my re-do attempts, I used WD-40. Went alot easier.

riotas

Good post. I would like to know where you bought the rebuilt kit
for the calipters? And are there kit for the fronts as well?
Thanks.

F18Rep

"If you wouldn't be comfortable licking any part of the piston, seal, or bore, ..."

Sam, you're just on a whole nuther level...;]

eclou

Paragon Products was the vendor I used. Very courteous and helpful

hosrom_951

Ditto

Charlotte944

I just checked their on-line catalog and the only caliper rebuild kits I found were for the single piston calipers for the NA cars.

Would you happen to have the part number handy?

Thanks!

DDP

Thanks for that post! I am about to do the same when I get my calipers powdercoated! Did you split the caliper when doing this? Thanks

Dan Hovang

Regarding splitting the calipers; in the Porsche 944 Factory Workshop Manual the following note applies to all four-piston calipers: "Never separate the two halves of each brake caliper from one another. Disregard any information stating otherwise, and alter any existing written documentation to this effect." Also there is a picture of the caliper with the bolts holding the two halves together marked and a note "never undo or tighten screws".

They are pretty clear on this topic I think. Guess they have a reason for stating this. Anybody know why?

Sam Lin

The caliper halves can be split. HOWEVER, their amount of clamping torque, plus heat cycling and stress from use, deforms them. They will not go back together and seal if you split the halves. If you split the halves, you must precision grind the mating faces again before reassembling. Very few people have the equipment or access to the equipment to do this - many entry-level machine shops don't! That's why it's much easier to just say NEVER split the halves.

Sam

DDP

Very good to know Sam, thanks.
I just spent a good 2 hours taking out the pistons. It is pretty difficult. I think most of my problem was because of the low pressure air gun. But, I finally did get them apart. Now if the more difficult it is to move them in and out of the cylinder, does that mean they are going bad. For example, one piston goes slides easily while the 2nd one does not slide very easy. Does that 2nd one have more a chance of needing to be replaced than the first? I guess I am complicating a simple question "how do you know when you need a rebuild?" Thanks

hosrom_951

I was going to replace the caliper seals and get the parts from the dealer.

If i remember, they were fairly priced (something like ~$40). So other vendors should have them cheaper.....

Dan Hovang

A good reson for rebuild is corrosion between the glide plates and the caliper which cause plates to bend and interfere with pad movement. Removing the plates can be difficult. The bolts are mounted with chemical lock which is supposed to unlock at 150 degrees celcius. I had to weld extra bolts on the head of the existing ones to remove the plates.

When glass bead blasting the calipers, remove the outer seal (dust cover?), but keep the inner seals and the pistons in place (fully pressed in). In Sweden the outer seals only cost like $60 per caliper.