Ed, with an old boat sometimes you need to do some exploring to find out what the PO left you. They might have replaced or rewired the pressure pump--and reversed the polarity or installed the pump backwards. Or there may be some type of check valve that has failed.
Best bet is to follow the lines from the tanks
, sketch out what you've got, and at some point the error will become apparent. At the same time you'll have inspected the whole system--which is essential is you want to rely on it. One bad clamp, one chafed hose, and you could wind
up with all the fresh wate rin the bilge