Originally Posted by bigcletus
But can the existing caliper be "un-stuck" ??
Yes you can, if the condition is not too bad. I recomend doing this as a temporary remedy.
You start by removing the caliper from the mount, if you don't know how-to do this, stop and take it to a mechanic. Have a helper slowly step on the brake pedal to push the piston out just far enough out so the dust boot is almost fully (but not quite) extended. You can then use a pick or small flat screw driver to gently lift/work the boot out of the grove that it is captured in (carefull not to tear the boot). Once released you will then be able to see the rust build up under the boot that prevents the piston from moving freely, you can use brake fluid and some paper towels to clean off as much rust as you can, do NOT use any tools that will scratch or damage the piston surface or the boot, I use an expensive popsicle stick. After cleaning the rust you can use a C-clamp to slowly push the piston back in, the boot should fall in to the groove as the piston slides back into the caliper body. Remount the caliper and pads, pump the brakes a few times to get a firm brake pedal, install wheel and take it for test drive, things should be alot better, if not you will have to replace the caliper or rebuild it.
I recall someone on here recommending Midland Calipers in Scarborough for rebuilt calipers,...props to the guy who recommended them. I sent a few friends with non BMWs there and they are happy with the service and product. They sell hard to find boots and hardware seperately as well.