I looked at my collection of leaking bags. The hole were close to crown of piston or located where the crown of piston hit the bag.
Plan: Make the piston outer shell smaller to allow more clearance between the piston and cylinder.
Previous version had 20 mils. Turning new set with 50 mil gap.
The pressure on the bag centers it in the cylinder but the piston can tip causing the crown to scrap the bag. To prevent that I am adding some diameter to the tail end of piston. For now I am going to use nylon screws that would also attach the piston outer shell to core. Later may use Teflon ring fitted to a groove at the end of the piston.
Status: Turned shells, still need to drill the holes for the attachment screws. Going to make another piston assembly for my piston test fixture. That way I can see if the piston twisting problem has been fixed.