How do you check brake calipers

[genphreak]

we had some cases especially on E32 750 with the H31 brake unit in our warehouse which were long time parked, that the brake calipers were stuck. The problem is the brake master cylinder, inside there are some seals and a bakelite ring, this bakelite ring 'swells' by age and then the master is stuck at some point. I know of some people which has disassembled the brake master and 'polished' a bit off the diameter of the bakelite ring.
Here a pic how it looks inside a H31 brake master cylinder, probably the others look same / similar

Amazing. What a find. I guess if you strip the master cylinder down and the ring is binding... its staring you in the face. eek! Can it be that the wrong fluid was used?