Re: Sticking brake caliper, 88 90 2wd

[eds%f9-a@Paoli.ATM.LMCO.COM]

> In a message dated 97-03-22 17:22:35 EST, you write:
> 
>  went to replace the pads today and cannot get the piston to restract,
>  even if I open the bleeder hose or go so far as to remove the caliper
>  from the hydraulics entirely.
>  
>  Yah, I can (and probably will) replace it, but does anyone have any
>  suggestions such as a bath in penetrating oil?

I can't say whether your car has threaded pistons, but as a former mechanic,
I haven't seen any on German cars up to about 1983.  My 4000S doesn't have them,
but it does suffer from sticking pistons every once in a while, in fact every
car I've ever owned (I usually own older models) has had this problem.

The only way to cure it is to either replace the caliper or rebuild it.
If you want to rebuild it, you have to get the piston out first.  My method
goes like this:

 1. Don't remove the caliper from the car.
 2. Remove the brake pad from the sticking side only
 3. If the calipers contain four pistons, insert a thick metal tool
    like a large screwdriver in front of the non-sticking piston to keep
    it from popping out for the next step.
 4. Pump the brakes until the stuck pistion frees up and pops out.
 5. Rebuild the caliper as normal.  (hone with emery cloth, replace seals)

It gets tricky if more than one piston is stuck, or if you want to remove all
the pistons (recommended).  You have to keep all the pistons from popping out
using big screwdrivers again, until they're all pushed out close to popping
out, but still keeping the seal so the pressure is maintained until they all
come free.  I've used up to three screwdrivers on the four-piston calipers
on  '76 BMW 530i before I got them all to pop free.  (They were stuck when
I bought the car.)

It's a hell of a lot easier to buy a rebuilt caliper and replace it, but
if you're handy, freeing stuck pistons is doable by us shade-tree mechanics.

Good luck,
Ed Skladany
'84 4000S