HiHiggins :laugh:
This is a test using 100 psi (689.5 kPa) air pressure which is injected into each cylinder via the spark-plug hole to determine the [amount of compressed air] leakage past the rings, gaskets, or valves in each cylinder of the engine.
what do you understand by a "leak down test"?
This leakge of pressure past worn rings / cylinders is called "Blow BY" and is the principal cause of crankcase pressure which forces excessive oil and oil vapor in the crankcase out of the engine breather hoses,joints or vents.----Keoke