HI FROGTWAT! [Great name!] The pressure check is made with the exhaust off, but if you want to get clever you could put the muffler on with some sealant, attach some tubing to the muffler outlet and blow into the tubing. If you cap off the intake, and make sure that the piston is down you can test the engine this way. If I'm lazy [which is always] I will attempt to fix a leaky backplate by spraying the area with brake cleaner, blow drying, tighten the screws snug and then wick some thin CA glue into the crack. Down and dirty. Aluminum paint can be tried to correct an imperfect seal on the head. You can try to hone a flat face on the top of the liner with a wetstone.A new head shim or gasket is the easiest thing to try first, of course. Over tightening the head screws can warp the head. I have a question FROGTWAT, do model engines run clockwise in AUSTRALIA?