Thread: Emptying an underfloor tank

put an airline into the fill hole, with a rag tightly bunged in (and around the airline) to seal it off.
Turn the air on and it will blow it all through.
If you want to syphon it out through the fill hole, then put a hose down to the bottom of the tank as well as the air line.
Works well for syphoning fuel instead of sucking a mouthful, Just blowing into the tank with another hose is enough to start a syphon.