Back to basics....
Test just piston & piston head in cylinder first
ptfe will work wonders but so will a new piston o-ring
getting a good seal is vital, getting a GREAT seal is not always easy though
piston heads themselves are often good to crap even after trying all sorts of o-rings n stuff
A lot of piston heads suffer a common problem that the 2 outer diameters on the head are not the same
often/nearly always the rear of the piston head is slightly smaller than the front part of piston
which imho is ar$e about face as the rear of piston head is where the o-ring sits & seals on compression
this means the o-ring itself is not quite as supported to seal perfectly when gun fires...
The front part of piston is the one that could/should be a smidge smaller as seal is not there but where o-ring sits
this also doesn't need to be great as the piston on retraction shouldn't be sealing perfectly at all or you could get suction or piston/bb suck
In effect you shouldn't get bb suck coz the o-ring should have some play sitting in the groove of the piston head
this allows the o-ring to fall forward, slight tiny gap between o-ring & rear of piston head allowing air to easily fill the cylinder on retraction
If this works effectively then by holding finger over cylinder and yanking the piston clean out of a full cylinder you should hear only a slight pop
If you hear a f*cking loud champagne cork pop then you got some suction - likely from an o-ring too thick for piston head's groove....
The perfect seal which is not easy to get everything as close to perfect will have very little pop if yanked clean out of full cylinder
and when testing for compression you will get strong resistance very quickly and with next to no possible leaks that the piston fights back so much that you hear a very light pop or pip I guess you could call it from piston fighting back and air slipping past o-ring and she comes backwards resisting the force of compression......
Blimey - that is a LOT of bollox even by my standards....
And I haven't even touched on subject of piston/piston head weights, bearings/spacers and f*ck knows what else
get the basics as best as you can - try another o-ring or two/dozen, stretch them but I personally would do any stretching on old o-ring not with lighter n crap in case you damage the o-ring, but stretch the sod over old cylinder, place in hot water - leave to cool and repeat with more hot water from kettle a few times
leave the sod say overnight - by morning it will be stretched properly like a pair of whore's drawers and won't return back to smaller size very easily
keep a few stretched o-rings to hand - you will find a time when you will need them
O-rings themselves can make a big difference - even same o-ring removed and replaced on the opposite way can make a considerable difference
then also roll o-ring so inner ring becomes outer ring, watch for bad seams on o-rings - yup just one o-ring can be installed say 4 different ways and the seal itself can be affected in this little area alone....
Yes some piston heads are f*cking crap with smaller diameters at rear, the holes may not air to pass well and swell/seal the o-ring against cylinder wall
so at best it is a lot of experimenting or trial n error to get just the piston head's seal as close to perfect as possible
(but some piston heads are just $hit no matter what you do)
Cylinder head can still leak but guess if you are still not able to see/hear the bad seal at cylinder head you can immerse the sod in cool/hand warm water and watch for air escaping around head whilst testing compression - or plumbers trick of washing up liquid around head and clock any bubbles
But to be honest you should know if head still leaks, you would feel compression loss if piston head/o-ring is good/great
heck you might even hear the air escaping blah blah blah......
after all is done to the best possible standard.....
drop the perfect o-ring in a bit of silicone oil overnight, polish cylinder with silicone oil and buff dry
reassmble next morning with spread of silicone grease and viola you should have a mofo sealing piston/cylinder head setup
if nozzle is $hit then replace with an o-ring nozzle of SAME LENGTH and hopefully nozzle seals on bucking ok
any other leak is going to be hop related but not in your nigh on perfect sealing gearbox (hopefully)
phew - this is short version of Piston Sealing Engineering at the University of Sad Badtids in London btw
ahh sod it I know there might be a few typo's here n there but my fingers are killing me and you get the general idea of what to check n do blah blah blah etc....
