Filling the cylinder with air is the only way to go, unless here is a valve that won't seal. Tapping on the retainer will loosen it from the keepers. The o-ring seals get hard and brittle with age and almost always break apart. Find all the pieces. It can be tricky to install the seals while compressing the spring and retainer and then the keepers. What about installing umbrella type seals, which install on the valve stem before the retainer. You will not have to worry about the o-ring popping out of place. Some worry about the valve stem not getting enough lube on the older sixes, but I haven't seen it to be a problem. Good luck on your repair.

The older I get, I only want to work on older rigs.