Cylinder Leakage Test
A cylinder leakage test is usually performed after a cylinder pressure test has indicated a problem. The cylinder is placed on top dead center of the desired cylinder and the spark plug is removed on that cylinder only. This is because when air pressure is introduced into the cylinder the piston will travel downward. The resistance from the other spark plugs helps resist this downward movement. 
Once the cylinder is on TDC air is introduced using a cylinder leak down gauge. These tools often have two gauges, one for inlet pressure and the other for leak down percentage. This second gauge tells you how much pressure the cylinder is actually losing. This loss is measured in percentage which is not to exceed 20 percent. Always check with manufacturers specifications.
If the leakage is above specifications the cause of the leak must be identified. Listen for air hissing through the throttle body or carburetor. This indicates a problem with the intake valve allowing compression to seep past the valve and its corresponding seat. Listen for air escaping through the tailpipe indicating a problem with a worn or burned exhaust valve. Also listen for air escaping through the oil fill cap indicating worn piston rings. Bubbles seen coming up in the coolant from the radiator neck would indicate a leaking head gasket.
