Pitting brake piston

brokke · 10-23-2002, 05:20 AM

Hi,

I am rebuilding the S-calipers of my 2.4S. The calipers were not frozen or leaking, I do it just as a precaution because the brake fluid was very dark. I know that in the ideal world any pitting of the brake piston is not allowed. But how serious are small pits. It is possible to buy new pistons but they are rather expensive (about 100 euro (that's nowadays about $ 100) for one piston).
Are there people out there who just used the old pistons with small pits without any problems?

Peter

Chuck Moreland · 10-23-2002, 08:13 AM

Can you post a picture?

brokke · 10-23-2002, 08:52 AM

Here are some pictures (I gave my wife a digital camera as a birthday present, handy..........)

Peter

Pillow · 10-23-2002, 09:00 AM

Hmmm, I would say no if it is real pitting. If there are very very minor pits that are shallow you might be okay.

Basically the the piston should be very clean and no pits to ensure a good seal with the rubber o-ring.

FWIW I have rebuilt some nasty calipers and the pistons always (miraculously) seem in great shape once cleaned up. Granted they get incredibly dirty in there! Why I do not know.

Also Pelican sells some hell-ov-a-nice rebuild kits - German ATE baby!

... On a dummy note, please remove your fingers from the middle of the caliper assembly when using an iar compressor to get the pistons out. I still have a friggin' knot on my finger from smashing it with a piston under 100 PSI... Probably a mircle it did not shatter the bone like a twig. 944 calipers are cake, 911 harder due to dual pistons. Use a 1x4 piece of wood in the middle to get both calipers out together.

On further thought, just get a new piston. Or a good used one from the Pelican Classified area.

Good Luck,

Chuck Moreland · 09:05 AM

The one on the right is fine.

The heavy pitting the left one is at the end of the travel. The pits won't come into contact with the seal until the pads are worn all the way down. Even then I'm not sure they will cross the seals.

Try sliding them into the caliper and see if the seal even hits the pits when fully extended. Full extension is the limit of pad wear.

edit: Since you can't see the seals when the pistons are installed, measure the depth of the seals and the distance from piston face to pits and compare to determine if they would hit.

Bones · Bones's Garage

i've just rebuilt my '79 SC breaking system with stainless braided Goodridge hoses and drilled disks......the right front caliper had been sticking so we stripped the pistons out ...after 23yrs there was a little gunge and light pitting only on the piston edge which isn't inside the caliper.....so we cleaned with fine wire wool and polished the pistons with Solvol Autosol......the brakes are excellent now (and look better)...i would say the piston on the right is certainly serviceable but the left one is scrap metal!......when you refit the pistons remember to make a template and fit them at 20 degrees to the horizontal......this is critical

Peterfrans · 10:44 AM

A rebuild caliper will set you back €149. For this money they´ll include new pistons, and an ATE rebuild set. I for one would not have pease of mind driving around with substandard brake parts.

The S calipers are notorious for corroding pistons, mine were just like yours.

brokke · 10-23-2002, 10:51 AM

Peter,

Who did the rebuilding of your s-calipers?

Peter

Pillow · 10-23-2002, 10:56 AM

>and fit them at 20 degrees to the horizontal......this is critical<

Why? Did I miss something?

... Sorry for the novice question, but in my experience it has not proved itself an issue.

In theory the piston probably rotates itself eventually into the spot it wants to seat the best I would think.

Bones · Bones's Garage

yeh Pillow 20 degrees.........i'm talking about rebuilding an SC caliper so it may well not apply to an earlier caliper.......

island911 · 10-23-2002, 12:22 PM

From this thread

Quote:

Originally posted by Roland Kunz
Hello

The angle is important to prevent geting a turn impulse into the piston. This will generate souns, uneven waer and the piston will spin tearing the rubberprotectionboot into pieces.

Grüsse

Pillow · 10-23-2002, 12:44 PM

I love you guys!! In a beer commercial kind of way... Not gay way.

I can only suspect now that the "turning" the piston can do does happen, but since the boot is fixed ends up ripping it to shreads.

The ATE rebuild kit instructions do not comment on this at all which is depressing. The Haynes does not mention this either.

... I might have to splurge for Wayne's book after all.

Thanks,

island911 · 10-23-2002, 01:26 PM

Seems I posted a pick a while back on the S-Caliper alignment; anyway, if you imagine the piston cut land as a pac-man, it would be headed to eat the bleeder valve.
Clear as Mud?!

Pillow · 10-23-2002, 02:15 PM

Island, you mention specifically the "S" piston, would this be different for the "A" or "M" calipers?

More stupid questions

Thanks,

island911 · 10-23-2002, 05:12 PM

Okay, I checked an old front SC steel A(?)caliper. The bleeder valve is more to the front, than the S. So the pac-man mouth is pointed between the bleeder valve and the line-in. Also, one side of the cut should line up between the mounting holes.

slakjaw · 10-23-2002, 05:16 PM

my pistons look alot like that a little bit of pitting and what not

i took some 1500 grit sand paper and cleaned them up

hopefully it will work

LATE
K

Bones · Bones's Garage

Quote:

Originally posted by island911
Okay, I checked an old front SC steel A(?)caliper. The bleeder valve is more to the front, than the S. So the pac-man mouth is pointed between the bleeder valve and the line-in. Also, one side of the cut should line up between the mounting holes.

Anyone rebuilding SC calipers should make a template as in this excellent article http://www.pelicanparts.com/techarticles/911_caliper_rebuild/911_caliper_rebuild.htm or face rebuilding your calipers 10mins after you re-fitted them!!!