Originally Posted by tdcarter72

The Duel Function Reservoir is located on the drivers side frame rail before the high pressure fuel pump and fuel filter.

If the tank readings are showing correctly on both tanks when you flip the switch, then the switch should be working correctly.

I had the same problem with my 87 F-150 and the in-tank fuel pump was the problem not the reservoir or switch.

Install a new fuel pump/sending unit in the rear tank.