More likely to be sticky/damaged/pitted pistons I'd of thought. After all, if the seals are perished/damaged they'll of let sh#t into the pistons. I've just had mine rebuilt by Bigg Red. They offer 3 different finishes and 4 colours. Only worked out at £50 labour on top of the parts costs. Here's a few pics of mine





Have to say I was impressed with them. Mine are done to their top option of finish with a polymer coating to resist dirt/dust/oil/brake fluid. Check out their website it's all on there.

cheeRS