Most (all?) float type fuel senders are really just variable resistors, with the fuel gauge measuring ohms.
I don't know without playing with one whether a zero gallon reading is zero resistance or infinite resistance, but I'd normally start looking for a loose connection, broken/intermittent wire, or a bad ground at the probe, because if any of that stuff happened, it's going to peg the needle one way or the other. If it's not there, then the next most common would be the probe itself worn out. This is easy to check by putting a multimeter on it, and measuring the ohms resistance. As you move the float up and down, the meter should behave just like a fuel gauge.
HOWEVER- I can't see how any of the stuff I just mentioned could be cured by cycling the master, which you said happened, so if you have old school gauges, I'd just swap them side to side and if the problem moves, then you need a new gauge.