Having rebuilt more hydraulic calipers than I care to remember, I have to say that Northwind is correct. You should be able to push the piston back into the housing with fingers alone – the fact you couldnt suggests the seals were sitting proud and were mashed by the piston as you pushed it back. Get a small right angled pick and run it around where the seal sits (Top and bottom if that makes sense) to remove any corrosion and then clean out with brake cleaner, fit new seal and try again :-)