if its both that are sticking- it might be linkage across bulkhead from pedal to servo that is siezing-up, have come across this on 2 occasions where vehicle operates in salt water conditions! if just one that's sticking- providing piston is free,if you look in calliper where calliper pin fits you will see that in calliper is a small stainless steel insert, if you ease it out you will probably find that there is a build-up of rust where it fits-this causes it to stick on pin,clean out refit insert and refit pin and "r" clip.hope this is of use to you.
k
August 2009