For mine, I used a 12 v automobile fuel pump that only pumps a very small amount per minute ( about 45-50 min. for 30 gal) thru a 5/15" line. I come out of the tank thru a small filter, to a 12v. 1/4" s.s. solenoid valve (from Grainger supply), to the fuel pump, to the main tank by a Tee into the 5/8" vent tube in the fill neck. The low flow does not interfere with the venting. As for controlling the flow and not overflowing, I don't transfer fuel until the main tank is more than 30 gal. below full.