Are you running an oil filter? If so does the line for it come off the same spot as the oil pressure line? Maybe the oil filter needs to be filled each time after sitting a while before the oil pressure will show on the gauge. With the drill spinning the oil pump if it just takes a few spins to get resistance It is probably not the pump. If it takes 5-10 seconds then it does sound like you are loosing prime. If this were my truck I would sure rather confirm the situation with another oil pressure gauge before removing the pan and pump. Just blowing air into the gauge doesn't tell you a lot.
Chuck
