[aesmith]

Seconded. There's a good chance that the stiffness is caused by corrosion of the alloy grooves that the seals sit in, and that corrosion will continue to get worse. What's needed is to remove pistons and seals, clean the grooves out thoroughly, then refit the seals making sure there's enough grease to properly protect the insides of those grooves. I like Silicon grease for this, others recommend red rubber grease. Don't use normal petroleum grease as this will cause the seals to degrade and/or swell.