ClassicCar

Technically known as the oil check valve. The '30 onwards pumps had no valves as such, just ports that were uncovered by the rotating oscillating piston, hence there is no oil pressure developed, as there is no resistance in the pipes which deliver the oil to the front and rear of the motor. It is desirable to have an oil pressure gauge, or as in later models an indicator light, so to develop some back pressure this check valve has a ball valve at the inlet, and a plunger in the top portion held closed by a spring. As the oil is delivered into the check valve it lifts the ball, and a pressure builds up because of the spring pressure, and this is registered on the gauge. As the pressure continues to build up it overcomes the resistance of the spring, and lifts the plunger, which blocks off the hole to the pressure gauge, and opens the port to the engine, and delivers the oil. The ball valve at the bottom is to maintain the pressure reading, otherwise the gauge would oscillate with each pump stroke. Hear endeth the lesson!