I would try using a 5 gallon can of diesel with a hose to your pump/filter head. This will eliminate any possibility of something in the tank blocking the pick up. If that works then you know it's forward of that.
I have a 4300 with a hst. I recently split it to reseal the front pump that is driven by the engine. Now when I push the forward pedal it moves in reverse. What did I put in backwards? Thank you