I don't fill the hose or have to play with the level of the gravel vac to get it to operate & I am using a waterbed pump (Its even the same color os the Lee's) and a 25' garden hose. The only time I have problems draining the tank is when I have air leaks. Make sure the hose washer is in place between the hose and the pump, & the hose connector is tight. Also check the fittings between the pump and the tank for air leaks. The "Python, Lee's, Waterbed pump," does not rely on a syphon for the system to work, and as Mighty Prawn mentioned it will even pull water up a short distance. Don't forget, they were originally designed to empty waterbeds and waterbeds were generally lower than any sink on the same floor. If there are no air leaks, the pump should work unless there is a kink in the pipe.
Other than an air leak, the only other problem area I could see would be the pump itself. If it was either cracked or deffective in some way that may stop it from working. A waterbed kit from wallmart is about $8 or so if my memory serves me and the venturi pumps are the same design as a Lee's or Python.
Hope this helps.
Chris
