A bleed down test uses a faux sparkplug or a simular fitting to allow you to pump the combustion chamber full of air on the compression stroke, a valve that can be shut off, to maintain the pressure, and a gauge that shows how much pressure the chamber can hold and (timed) how long before it all leaks out. Ostensibly more accurate that a comp test. The simple test you are referring to, is still a good first start. Your looking for your readings to be within' (I'm guessing here) 3-4 lbs. On cars motors, the books usually say 5 lbs max variation.
