Cylinder leak down testers should be one of the first things you reach for when wanting a physical check for engine condition.

The leakage can be determined by the following observations:

1. Hissing from the exhaust indicates exhaust valve leak.
2. Hissing from the oil filler cap, piston rings or worn cylinder rings.
3. Bubbles from the radiator cap, head gasket or crack in the head.
4. Air escaping from an adjacent spark plug hole, head gasket.
5. Air escaping from the carb/induction, inlet valve. 

18mm Adapter / 12mm Adapter / 10mm Adapter

This test will give you a far better insight into the integrity of the engine, it is possible to have a good compression test and a complete fail on the cylinder leak down test. The leak down test is the one to trust!