Having not changed one of these pumps in living memory i think that.....
1 some one is leaving the pump running all flight or.
2 the pumps fitted may be OHC and not new.
3 i think that the chances of a pump "drying out" are unlikly unless the the aircraft is not flown for weeks on end.
I would first sujest that first you check the seals on the tank filler caps and the lugs on the bottom of the caps for slots worn in them ,this will eliminate most of the chances of water getting into the tanks.
If you have to replace another pump i can recomend buying it from airpart supply at booker (01494 450366) if they supply a new pump you can be sure that it is new and not OHC.