Whether your pump is positive or non-positive displacement, providing the best possible conditions at the inlet is vital to reaching maximum pumping efficiency, as well as prolonging the life of your pump.
Let's start with the reservoir itself. Take a good look at the breather cap on top of it; that cap is very important! In good condition, it allows air to flow into the reservoir to make up the pump draw. Without it, the pump would create a vacuum condition along the inlet line and reservoir.
There are a few ways to cause a starved inlet (plugged breather, too long of an inlet hose, poor placement of the reservoir), but they all result in the same problem. The pump is drawing in less fluid than it is trying to displace. Not only will a pump with a starved inlet not be able to put out much flow; it is in serious danger of cavitation.
The opposite of a starved inlet is a flooded inlet. A flooded inlet occurs when there is more than enough liquid at the pump's inlet, to the point where it's even holding a positive pressure against the inlet (1.2 psi is about ideal). As the pump turns, fluid rushes into the pumping chamber, keeping it fully charged.
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