Keith if they are o-rings you may be getting the wrong hardness. O-rings are measured in a hardness rating of shore A durometer. If you have pieces of the original rings, take a piece with you to try and match the hardness. A good industrial seal vendor should have a device to measure the hardness. The material makes a difference too. Most times it would be Nitrile and you shouldn't be using anything less that 70 durometer. If you have 70 duro now, then try moving up to 90 duro Viton. They will cost you more but last longer.
OK, that said if you are not the original owner maybe the original seals were not o-rings and were somethinge else. O-rings are not the best technology for reciprocating seals. If they are piston seals and you have the piston out you may want to get 70 duro Tee seal sets. Tee seals are designed to use as reciprocating seals and the have back up rings to help close up the extrusion gap between the piston and the cylinder bore. You can convert to tee seals IF they were originally o-rings but this would mean taking the pistons to a machine shop and widening amd deepening the groove a bit. Stay with us here and the forum will help you solve your problem on way or the other.