don't listen to mtelliot (not meaning to make enemies).

Use a block of wood and suitable spacers and take both sides apart and rebuild them together. If you fix one side and use penetrating oil on the other side, the penetrating oil will inevitably migrate onto the side with the new O-rings and bugger them up.

I searched the postings and found they all suggested using air which works fine if your pistons aren't as badly siezed as mine were.

Uncle:

Good point. But, if you stand the caliper up so that the oil penentrates into the piston, then there's no risk (you would want to do that anyway). You're going to want to wipe the cylinder out anyway.

Point being, there are a lot of opinions on this board. Find what makes sense to you, and try it. If it doesn't work, trying something else.

Compressed air seems to be the common way to push them out.