On the old cylinders with White PLASTIC PISTONS,, have an injection molding flange , at least mine have. one of then leaked.
I fixed BOTH, by removing the piston from the cylinder. chucking it up in my lathe, spinning it and running a micro file in the groove removing that flashing.
you can do this several ways including spinning them in a drill and using 1500 grit sandpaper rolled up.
You don't want to make the groove deeper, you just want to smooth the flashing. If you don't remove it it doesn't matter what you put in the groove. it will blow out.
mine were double o ring. They have held air sense September. just touch the seat of the groove.
I know all of the reasons we shouldn't have to do this , but this is nothing New , This is R/C, Those of us in this for the last 45 years, have had to fix dam near everything we have had. especially all of those years ago.. As the saying goes, you get what you pay for... well maybe. On the other hand some of this stuff has been taken to a new level.