The main fuel outlet draws from a tube sticking up from the petcock, reserve from the base of the petcock. The reserve fuel is whatever is below the tube top. The tube can crack or be taken off, the valve disc can be torn and leak from reserve to main.
You should de-rust the tank. Lots of stuff will do that, phosphoric acid is what I use: cement floor pre-painting etching liquid from Home Depot is cheap and powerful.
After derusting - whatever you use - you have to rinse out the chemicals with water then dry the tank (and oil it if you don't use it right away). Rinsing with pure methanol (gasline antifreeze) will soak up the remaining water, and the remaining methanol will evaporate quickly if you warm the tank a bit (no open flame for that of course!!!). Sloshing a bit of 2-stroke oil around will oil the inside and stop rusting in storage. Normal motor oil is OK but I have 2-stroke oil for a chainsaw and figure it was made to burn in a motor...
Tank coatings (POR-15 or whatever) is not needed except to seal pinholes or a fibreglass tank (ethanol eats fibreglass). The tank was not coated when new, and in normal use will not rust. If you do get water collecting in the tank bottom, use some gasline antifreeze occasionally - but just keeping the tank full will usually eliminate any water buildup. Hopefully you aren't getting gas with water from the pump!
