Having the holes above the final position of the piston sounds like a better idea than having them below the final resting position. The reason we have them there at the moment is to let the air around the top of the piston move out quicker when I activate it. If I put the holes in the top then I suppose I can get it tested just like any other piston. We use a separate valve for exhausting the CO2.

Thanks for the advice.

Dave