I have to make a correction to the above procedure. You have to pressurize the cylinder at top dead center because when the piston is at bottom dead center the valves are on overlap.
Also, if you try this, you need to block the crank from turning because if it's not exactly on top dead center, it will start to creep and then it goes right to bottom dead center in a hurry. You could get hurt or the valve will drop into the cylinder.
If you try this method with the air note that you have to remove the Schrader valve from the end of the compression tool hose. ***** ALSO, a big lesson I learned is that the Schrader valve is not a standard Schrader valve you find in tire valves. The spring in the compression tool hose Schrader valve is a much weaker spring to allow compression from the cylinder to enter the compression gauge, so if you've ever used a compression gauge tool and the readings seem to be really low just know that you need to change out the stupid Schrader valve before throwing out that perfectly good tool out. You'll need to buy the special Schrader valve specifically for compression gauges. They are available.
