Why are you fitting an NRV not needed if all is well with system. If you are getting air in clear pipe it is due to air leaking back through spill pipes or air leaking past O rings either side of filter.
Cheers wammers, I was wondering if the nrv should be there especially as I have no indication of air in the system, last owner must have done it as he probably could not be bothered changing the in tank pump.
Davie