Air Lock and Vapour Lock

An air lock is a pocket of air trapped in a system that should be full of liquid. In a hydraulic brake or clutch it makes the lever feel spongy, because air compresses and fluid does not. In a cooling system it can stop coolant circulating and cause overheating.

On two-strokes with oil injection, letting the oil tank run dry lets air into the pump line. The pump must then be bled before riding, or the engine may run without lubrication and seize.

Vapour lock is a related problem: fuel in a hot line or carburettor boils, and the vapour bubbles stop liquid fuel reaching the engine. It shows up as stalling or poor running in heat and usually clears once things cool down. The cure for an air lock is bleeding the system properly.