Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

You can try putting 20-30 psi air to your boiler to check for leaks. Have some soapy water to pour on all fittings mated to the boiler.
If you can get a hand pump, you can fill the boiler with water to the top and then pump more in with the hand pump. First bring it up slowly to operating pressure of 60 psi or a little higher to determine if the safety relief valve is functioning. If all is good at that point, remove the safety valve and install a plug and a higher reading gauge. Then take it to 1.5 times operating pressure or 90 psi. (Do not try this with live steam or air). Then you may want to take it to 2X operating pressure or 120 psi. The 504 is a very stout piece of hardware and should likely withstand far more abuse. As stout as it is, a hydro test is still recommended. This way failures of any kind are harmless. If something gives you only spill a little water and it isn't hot.

Best of luck with it and I am glad to read that whatever kept you away from it has allowed you to take another crack at it.

Gil