Your methods are sound but you're not really solving the problems you're coming across (ie: using air seal grease? there's such a thing?...)
As a general rule, if anything is qualified by the phrase "actually not bad", then you should probably revisit it until it is "excellent" or "bulletproof" - you are after all diagnosing an airseal problem.
Did you check to see if your piston is binding against the piston rails?
SCs Top Tips - Piston Check - YouTube