Many high performance water pumps are not self priming. They rely on a one way valve called a foot valve to both prime the pump and to hold that prime when the pump is not running. The foot valve attaches to the intake end of the pump hose and also incorporates a screen to help prevent debris from plugging the pump. A pump is never primed while it is running. The proper procedure is the agitate the foot valve back and forth underwater in a plunging motion in line with the hose itself. This action slowing forces water up to and into the pump, driving out the air ahead of the water. Attempting to prime a pump by pouring water into the pump output port will often result in an airlock, so the plunger method is generally more reliable. Continue with the process until water is observed coming out of the pump output port. Then and only then do you start the motor, and if water does not start pumping immediately (within a few seconds) something is wrong. Shut the pump off and double check everything. Once you have a pump properly primed backpressure from water in the system should keep the valve shut and hold the prime when the motor is shut off. Insure that the intake never rises above the water and take care to place the intake where it will not suck up excessive sand and debris. Most of our foot valves have a marlex body with plated intake screen and rubber flapper valve.

This explanation and the attached video from the Keene Engineering literature should help:

“Before starting the engine, the pump must be fully primed. This means the pump must be full of water and all air removed. All jetting pumps provided with our dredges have a mechanical water pump seal. Without the presence of water in the pump, friction could cause a seal to overheat and require replacement. Priming the pump on some of the smaller models is accomplished by thrusting the foot valve back and forth under the surface of the water in a reciprocating motion. This will pump water into the foot valve assembly and into the pump. A pump is fully primed when water is observed flowing out of the discharge end of the pump. It may sometimes become necessary to hold the discharge hose above the level of the pump to complete the priming operation. The larger dredges that have a rigid foot valve, are easily primed by removing the cap provided on the foot valve and filling, until water overflows. Caution must be exercised to prevent sand from entering the foot valve or intake portion of the pump. Excess amounts of sand could damage the water pump seal, or pump impeller. It is recommended that the intake portion of the foot valve be placed in a sand free environment underwater, such as a small bucket or pan.”