0 f/p new airdog2 165

i just installed a airdog2 165 and a sump pick up and i adjusted the f/p to around 45psi maintainend around 40 for about 30mins and now the gauge doesnt even git above 30psi on start up then drops fast to 0 but starts up fine.the truck been setting for a a couple years and i had a raptor 150 and the gauge stay at 45psi at idle with no issue .the gauge is a autometer digital display .could the lift pump took a **** that fast ? or the sender for the gauge? and i also have a tork tool ofv which has been in the truck with the old lift pump