All you need to do a leakdown test is an old spark plug and an air compressor. File grind, or hacksaw away the crimp of the steel plug shell so you can knock the porcelain out, and either use a 1/4" pipe tap to to thread the shell for an air line quick-connect, or solder or braze a connector into the shell. Once that adapter is fabricated, just connect an air line to the cylinder with it and apply air pressure with the transmission in high gear and the wheels chocked. You'll need to have both valves closed at the time- - - -the simple way to do that is point the distributor rotor to the cap tower for whatever cylinder you're testing. Listen at the carburetor, the tailpipe, and the oil fill cap to find where the pressure is going. Also, check the radiator for bubbles- - - -indicating a leaky head gasket and/or a cracked head. That test is simple, and it tells you a lot more about the engine condition than a compression test, or even a teardown, sometimes.
Some compression testers have a quick-connect fitting that can be adapted to the air hose. In that case, just take the Schrader valve out of the end of the compression tester hose and you've got a leakdown adapter!
Jerry
