The spring I used was a Titan from J Knibbs, and there is a thick steel washer in the breach end of the piston. The piston seal is home-made from some shoe leather, soaked for a week in Neatsfoot oil. It had a new breach seal and I lubed the compression chamber and spring with a 50/50 mix of LT2 moly grease and lithium grease, liberally applied.
I've found old springers with leather washers like to run 'wet', with a nice puff of smoke lingering in the barrel after the shot, although not too much or you get detonation and poor accuracy. I also put 2 drops of motor oil down the transfer port after each tim of pellets, and leave the gun standing barrel up over-night.
As you mentioned, the mk1 trigger was never the best bit of design, but it is no worse since the rebuild than it ever was - heavy but predictable. I can still get 1" groups with it at 30yd on a good day!