Sill having problems with the rifle

I have taken off a bit of the materiel on the leather washer (enough so that the piston can be easily moved up and down the barrel by hand)

It seems to work fine when there is no pellet in the chamber but as soon as i put one in the rifle does the same as before (piston slowly moves down the barrel).

I might just put the old original piston washer back on. Yeah it is tatty, but i know it works ok with it on.