Pressurising the cylinder tells you there is a leak somewhere, and with a good crank seal and rocker gaskets the only way out is up the scavenge pipe, through the pump and into the oil tank. It is what you would expect, dry pistons and bores will always pass air at high pressure. If the head gasket has failed, there is also a theoretical path to the sump via the pushrod tunnel and cam followers. If the test is repeated with oil in the bores and the leak lessens, it is more indicative of a bore sealing problem, rings/piston/bore.
To check the head gasket involves almost as much dismantling as removing the barrel to inspect the bores, pistons and rings, so you may as well go the whole hog for the sake of a new base gasket.
Swarfy.