Hello!
Im not so well versed in c2s but ba57stards is i beleive.
However, i guess they would be standard fair. Put an adjustable spanner or cable tie around one piston and pump the lever and the unclamped piston should pop out. Use a pick to get the seal out.
When putting in new pistons or seals, i fill the lid of my fluid bottle and let them sit in there to coat them.
Put new seal and/or piston in, you may have to walk it back with the round end of an 11mm or so spanner.
Then clamp the replaced piston and repeat for the other side.
Thats how i would do it.
You may find its leaking from the split in the caliper? Not sure if those alloy bolts you have in there would cause any problems.