Open Valves Makes it Simple
We have spec'd our units to have the Tank to Pump valve open in the IN position for the last 50 years with out problem. The basis of our decision was that most of our operations start with tank water and this change in valve handle configuration removed a potential mistake. ( I think someone screwed up badly on a call to start the discussion) In addition, since the pump is full, we do not have to hit the primer - the pump is ready to go. Just increase throttle and open line and water is moving.
For positive pressure sources the check valve will keep the tank shut automatically. We routinely close the valve in this case to avoid intermittant loss from the tank if the incomming pressure gets too low to assure there is water in case of a total loss of the supply line so we can get the interior guys out.
So the only time we really have to pull a valve is when we draft - about 2-4 times a year on calls
The downside is a the potential for a BIG puddle under teh truck. If there are maintences issues with the packing or leaking valves this can be a problem since you can drain a significant amount of water between calls.
We are located in the Philly suburbs (near CE 11) so freezing is not a problem during the drive to the call.