Gary:
If you are converting from a pressure tank to a suction system, the
pressure in the tank should be atmospheric and I would think no air
pressure valve would be needed....As to the pressure from the inlet hose,
the check valves in the pump keep that pressure from going into the tank...
Turning the pump on while connected to a water supply usually does no harm
as the pressure in the supply hose is enough to keep the pump from coming
on... If you had a real low hose pressure and no check valve in the inlet
hose fitting line, you could pump all your tank water out into the supply
hose... Check valves in the system are supposed to prevent that, but
sometimes the valve sticks and you tank water piddles out on the ground
when the pump runs... I solved that in my trailer with a fitting with a
check valve that replaces the hose connection... I always use a pressure
regulator at the supply faucet of the park which protects the hose and the
trailer... Pressures as high as 90 + psi can be found in some areas...
Usually more that the trailer fittings can handle... I am sure you are
aware that when converting to suction system, the pump is moved to the
delivery side of the tank... sucks the water out of the tank and
pressurizes the trailer....to prevent a vacuum in the supply tank it's
vented to the outside...Filling the supply tank from the water hose inlet
connection usually means a special valve that operates a bypass around the
pump... A manual operation where you operate the bypass and watch the tank
level and shut off the bypass when you have the amount of water you want to
carry...I have not yet figured out where mine is if I have one and I
continue to fill with a separate hose through the little trap door inlet...
Good Luck ...
Jim