Most transfer pumps — pumps that move liquid from one tank to another — are self-priming. However, there are some that require priming to get started. The reason pumps sometimes need priming is because they function using the principle that water escapes from high pressure areas to low pressure areas. If the pump isn't primed, then the pressure isn't high enough to pull the more-pressurized water through. This is why it is necessary to increase the pressure with outside air.
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Close the fuel line supply. There should be no connection between the transfer pump and its destination when you are putting the primer on.
Push the priming pump onto the closed fuel line.
Bolt the pump on using the attached bolts and gaskets.
Press the fuel pump down a few times to increase the pressure in the transfer pump.
Close the fuel line and remove the pump. The transfer pump is now primed and ready to function on its own.