hi wadcutter
cant remember where exactly but if you follow the pilk gun article and read all the comments someone there come to the same conclusions as me and filled it with too pack filler and had the same results as a fully repaired one ,remember this article comees from mid 2000 ...so i reasoned that filler is not as hard as tool steel and i could knock it out if necessary with a small oring on the front to fill the space and one in the second groove...i left the rear one in place its important to make it flat across the top to start with....

the trigger seal....on a spannish site they used an o ring....and reported 1 hours staying loaded... i could not get one that was a good fit and lasted more than 50 shots
then made a punch by chamfering the edge of a piece of pipe, to punch out flat washers.....make a mark with punch then use suitable pointy thing down middle of pipe
to mark the center.. i then use a multi hole punch like pliers with rotating punch sizes ....use the smallest then punch out seal
when the gun is ready to go into the stock there is a small allen key to adjust the pressure on the seal when cocked i would instal and fire it 10 tines( no pelletts to warm it up and adjust again you will need to do this every so often . i have tried other materials but they either didn't work or disintergrated after a while......did think about making one out of leather but you would have to take it out to oil it fairly often....
you can adjust the length of the cocking mech at the trigger end to control the cocking of the trigger there is maybe 5mm of adjustment
if your really worried about the piston,then
im in brighton on the out skirts 400m from the end of motorway
come and have a go on mine then we'll open it up and you can see what i have done
cheers ian