It's from here, the back side of the 3rd stage. Is there something I can use to help seal that up. I'm scare to put a dab of oil or something, no idea what that would do..
View attachment 661738
Er that's the second stage piston side you have in the photo (not the 3rd stage)
Its called "blow by" gas leaking past the ring on the compresion stroke.
Two causes, The ring is worn, or more likely the ring was set in badly in the first place and a small "Leaf"
edge has slipped out of the piston groove and is not allowing a full pressure seal.
The other option is the O-ring ( a big fat "worm" of a thing) under the compression ring has worn on one side or has been scuffed away.
Solution:
Remove the compression ring and either relace it or repair it by cutting a new feather edge on the end of the spiral with a pair of sharp, very sharp scissors.
And while you are at it remove the big fat o-ring (Part 123-313-5) and replace. Cut the old one out by using a pair of needle nose pliers to squeeze up the ring and cut it off without scratching or damaging the bronze piston.
To replace the 0-ring boil it first in a kettle for a few minutes to soften the ring, poke it out and dry it on a paper towel and fit it on the broze piston as soon as possible before it goes hard again.
Then you can use the remaining water in the kettle to make the tea.
Also when fitting it dont twist the thing or roll over one side. The o-ring needs to be centred at the centre line and not twisted or you back to where you started from.
Service 101 to us lot I guess but a whole heap of pain to you lot if you mis fit the assembly.
Also important before you ram the piston into the liner dont shove it in, be gentle turn it slightly and push slowly. That piece of advise was brought to you from the womans section of Cosmopolitan Magazine and reccomended by Mumsnet.