The $100 challenge.....

Yes, i'd be keen on the link for hardner. 

Paul, I'm wanting metal to metal contact at the breech with a fully compressed nitrile or butyl O-ring.  I'm not sure if this is the correct thing to be doing but I'll give it a go.  My  piston isn't terribly concentric and I compressed and tack welded the top joint to get it a bit closer to round.  I'm going to button it anyway (I've figured out how to do this without wasting too much of my 30mm PTFE rod) which will overcome this.

For the trigger I silver-soldered the part on.  Not a lot of heat required, just gentle application of a propane torch to the area, with tiny pieces of the solder strategically placed and some flux.  I filed the black coating off the trigger and used a long screw to hold the brass piece in place with Blu-tack which surprisingly didn't liquefy with the heat.

Another issue I want to address is end of the slot where the cocking lever engages with the piston.  I started to feel some metal to metal contact when cocking the gun after the first set of mods and decided to pull it back down to check.  No damage to the cylinder but the soft steel of the piston had pushed out in two places where the lever presses against it.  I've cleaned that off and squared the hole to better match the lever end and I have some Kasenit (direct copy apparently) case hardening powder on its way from Croatia via Ebay.  I'll use that on both items to hopefully correct that issue.  Kasenit doesn't appear to be available any longer and the Cherry Red alternative that you can buy locally is ridiculously expensive.  If anyone wants the link please say so.