A leak-down test also measures pressure in a hot engine, but instead of using the engine to compress the air, you supply your own with a compressor. The testing tool sets the incoming pressure, usually at about 80 psi, and then tells you how well the cylinder holds that compression. Most importantly, it allows you to easily determine where the compression leak is. You can move the piston up and down in the cylinder, and thereby find a defect in the cylinder wall if pressure drops at a particular point in the piston's stroke. More likely, the problem will be worn or stuck rings or a poor valve seal, and you can determine which one by listening and with the addition of oil. By listening, you can determine where the pressurized air is escaping. If you hear it through the exhaust, your exhaust valves are not seating properly. Escaping air in the carb indicates that it's the intake valve(s). If you hear the hissing of escaping air at the oil filler or crankcase breather, the problem is probably stuck or worn rings.