i have had my 850 since 1990 and has been kept under cover for most of that time, before that it sat in a garage in Burbank California so it has had a relatively easy life. For the last year and a bit the front brakes have been a bit hinky when I have driven the car. The front brakes have never been touched beyond a new set of pads. A few years ago I bought new pistons and a rebuild kit from a vendor in Germany anticipating I would need to redo the calipers. I pulled the parts out today as the right front caliper was effectively locked. Pulled the caliper and used the brake master to push the piston out. Spent a long time cleaning the caliper up particularly the groove the seal lives in. The calipers are aluminum and have stood the test of time fairly well, the pistons were somewhat pitted so it was time to use those nice new pistons. The old pistons were chromed steel and thus could and did rust in spots. The new pistons are stainless steel and appear effectively the same as the OE ones. Until you go to install them. One of them I was able to install but as I rotated the part in the bore you could feel the steel galling on the bore of the aluminum caliper. The second one I couldn’t even get halfway down the bore. The old pistons when I measured them were 44.87mm but the new ones were 45.1mm. Seemingly a minor difference but one that apparently is keeping me from being able to use them. The parts are very nice replicating all the features of the existing parts aside from the diameter issue. I ended up cleaning up the existing pistons with a brass brush. The calipers are now working properly, the new seals in combination with the less than ideal pistons remains to be seen. As so often happens when dealing with old cars a one hour job ended up taking most of the day. As an aside I did have a brake hose that was not allowing easy movement of fluid with in it. I replaced it with a new one I had on hand. Tempting to stick the parts onto a lathe to reduce the diameter of the pistons.