Not critical or important. Both brass nipples go right to the top of the tank. The nipples are not hooked up to anything on the inside of the tank.
The nipple that goes to the header tank is a suction line, which keeps the UAT full by drawing fuel from the header and mains.

Make sure you don't allow any air leaks at the hose connections at any of the three lines. Also, check the screw on cap for security occasionally. Any air leak at the UAT will prevent fuel flow into the UAT.

I would suspect that you have an air leak at the cap or at one of the UAT connections. If the cap is leaking, reseal it with teflon pipe thread tape.

I sometimes install a Festo 6mm ball valve on the end of my filler line, or use a Festo one way check valve in place of the aluminum plug supplied with the UAT.

Harley Condra
BVM REP
JetCat REP