I have the same kinda problem with my Triton LP Thermostatic - which uses the same kind of cartridge that slides into a brass housing. I've had it about 20 years now and replaced the cartridge twice before, easily, with no leaks. This time I can't get the cartridge to slide in smoothly and be leak free. It seems that 'common wisdom' suggests that poor O-ring seals are 'rolling' out of their grooves as the cartridge is slide home and that replacement seals should be tried. I'm now on my third set . . .
Kepla
April 2010