Water treatment??? Do you mean a fuel dryer for gasoline. Bad, very bad.
my 2 cents, old buses have been known to have collapsed fuel line, and I have seen a few with a fuel line tied to the side from the tank to the engine. It has happened to a few here, also fuel treatments can dislodged crap and block stuff before it get to the filters. Some have lightly (3-5 psi?) pressurized the fuel tanks to blow the lines. If you do this, disconnect the line before the 1st F filter (closest inline to the tank) put an air line in the fuel fill neck use a rag or whatever to block the fill neck a lightly pressurize the tank. IT IS NOT A PRESSURE VESSEL ONLY ADD A LITTLE PRESSURE, DON'T LET THE TANK SWELL OR RUPTURE.Fuel should flow out the disconnected fitting iadd a air line there and into a container you remembered to put there. If no fuel comes out, you can try back blowing the line to clear it using air or fuel under pressure, then pressure the tank and try again. Good luck, dress warm, keep it simple....Sounds like no fuel to me, plenty of priming in your future, check the threads re priming etc...If the line is collapsed it will flow fuel under pressure but will not flow fuel being sucked so that can be confusing.....Keep it simple.