All a leak down test needs is a source of air pressure and an adapter to put the pressure into the spark plug hole. I use an air hose quick coupler threaded or brazed into a spark plug shell. If the air source has a pressure regulator, set the "deadhead" pressure with no air flowing to 100 PSI. When air is introduced into the cylinder, any pressure drop represents a percentage of leakage- - - -90 PSI = a 10% leak rate. Either lock the crankshaft with the piston at the top of the compression stroke with both valves closed, or disconnect the pushrods and let the air pressure push the piston to the bottom. A "Top of the cylinder" test is better- - - -that's where most of the tapered cylinder wear is located.
Listen at the carburetor, tailpipe, and crankcase filler for air leaks, which will pinpoint the leak to intake or exhaust valves or worn piston rings. That's something a compression test can't do. Bubbles in the radiator indicate a blown head gasket and/or a cracked head or block.
Jerry
