Old thread, but have data now.
The Northern Tool aux tank shutoff valve, I would NOT recommend it.
I bought one, installed it and ran it for about 6 months including a 3000 mile summer vacation towing a 13k 5er.
The transfer rate is very very low. It will NOT keep up with my DD2's and towing a heavy 5er at 60 mph. It will barely keep up with running empty at 60 mph.
Today I decided to take it out and take it apart and find out why it would not keep up with DD2's.
I feed the OEM tank from a aux bed tank with gravity feed through a AN-6 hose to the Northern Tool valve. The fitting on the Northern Tool valve fits INSIDE of the AN-6 line, but still has about 1/8" ID for fuel to flow through (which should be enough).
Then the Northern Tool valve has a automatic cutoff float in a small barrel, but has 1/8" drain holes drilled into the small barrel. I cut the automatic cut off float mechanism off to see what was inside.
AHHHHHH HA, found the culprit. In the top of the automatic cutoff float area is a little less than 1/16" hole with a chamfer that the automatic cutoff float ball rises up into the chamfer to cut the fuel flow off when the OEM tank gets full.
Sooooooooo, you are gravity feeding fuel (not under pressure) through a less than 1/16" hole into your OEM tank. I think that is the problem, the feed hole is not big enough to allow enough gravity fuel feed to reach the tank.
The probably designed it that way with a very small float seat hole because the float ball is about 1/4" in diameter and probably could not work against much gravity fuel flow as the OEM tank gets full.
Sooooooooo, I do not recommend it because of not enough gravity fuel flow because of a very small hole in the valve seat for the gravity fuel to flow through.
Bob Weis