Not sure if this shows an underlying issue or not (probaly does) but to prime water pump
Switch filter value off
remove NRV from pump and reattach imput pipe
open filter value and allow water to backflow down input pipe (to prime it) (loosen pump prime valve to let air out)
run pump a few cycles
turn pump off, close filter value and replace NRV
open filter value and prime pump
turn pump on.