Even the Velocette is not foolproof though. The check valve in the pressure pump feed line takes the form of a ball on an inverted seat requiring a spring to hold it closed. Pump suction once the engine is running defeats the spring and allows flow from tank. The suction will only be adequate to achieve this when the oil lines are full however so always a little worry after an oil change, although generally only a concern after a rebuild when line must be primed.
