I have the exact same problem with my KMP retracts, The piston does not seal on the inside of the cylinder.

What I did was use the pistons out of a wrecked set of robart spring down retracts. the ID of the cylinder is the same as the KMP cylinder.
The threads in the pistons had to be re-tapped to 3mm for the KMP shafts. but my system will hold air overnight in both positions now.

This was the last set KMP retracts I buy. I would gladly spend the extra coin on sierras or something just to avoid all the headaches I had with these crappy retracts to FINALLY make them usable.