Loctite 271
wash the cylinder with degreaser then thinners...as clean as you can get it..i have washed the cylinders in the dish washer before now...just do not tell the wife
make a plug for the transfer port
drip in loctite and make sure it runs around the end plug inside...all the way round, try to not get it on the cylinder walls
install piston with seal and push till around 5mm from all the way home
plug transfer port from outside
withdraw piston to create vacuum and hold it back by having string on the latch rod and wind it round some wood etc...the trick is to hold a high vacuum in the cylinder....the cylinder should be upright with the breech at the bottom also
leave it over night like this
In the morning, release the piston, withdraw the TP plug, withdraw the piston, get some doweling 25 to 28mm dia, making sure the end is flat and true, spray glue some 240 grit sand paper to it...do NOT have the sand paper on the sides, just the face...spin this against the breech face to clean off the excess loctite. Renew the paper till the breech face is shiny, now attach some 400 grit wet and dry and polish the face of the breech, again do not let the sand paper touch the sides of the cylinder wall.
blow out with an airline or air duster , rebuild the gun and test
if you have an old gun with a 4mm TP, if you install a synthetic seal it may go slammy, if it does you need to sleeve the transfer port down to 2.8mm or so