We usually did the air pressure test on a dedicated fixture in the machine shop, on a bare cylinder head. All the coolant ports were sealed off with clamping devices, and air pressure was applied to the head with a paint gun regulator. As long as you don't overpressure the radiator or the heater core, you could probably do the same thing by pressurizing the complete cooling system after draining the coolant. An atomizer bottle with a mixture of dish washing liquid and water, or a bottle of kids' bubble soap sprayed near the crack provides the leak detection. Another method would be to disconnect the radiator hoses and splice them together with flex hoses and a piece of exhaust tubing- - - -ditto for the heater hoses, and apply air pressure through a Schrader valve soldered into the radiator hose splicing fixture. You could also adapt an air line to the drain petcock hole and use a pressure regulator. That would take the radiator and the heater core out of the mix and allow more air pressure to be applied. Good luck!
Jerry
