all the piston heads I have are the same as yours Louis and as you've seen this type are rebuildable but usually they were rebuilt by the service centres(you can get them from JG on an exchange basis still)
is the rod adjusted correctly and these seals are designed to squeeze a bit on opening and expand when pumped so they sealed to the tube
how much more clearance do you need can you shave a bit off the piston pivot block so the lever opens a touch more
on one of my 140's and a Sheridan I found I had to slot the breeth hole a fraction when fitting upgraded pistons