It depends on how full the tank is. The outlet pipe is in the shape of
an arch inside the tank over to the center bottom pickup point. Once you
clear the fuel out by back flowing air it can only fill the arch and
drain out by gravity if the fuel level inside the tank is higher than
the high point of the arch. Once the arch is filled with fuel it will
then be able to drain the tank all the way due to the siphon effect.

Mark

Don Hanson wrote:
>
>    Here's the question:  When I took the supply  line from the tank to the
> pump off before I put the compressed air (50psi) backwards into the fuel
> tank, the gas flow out of the disconnected hose was very
> free-flowing....and now I get barely a dribble out when I unplug that
> hose...When I put the air into the  hose, I took the gas cap off and I
> could hear the air going into the tank...lots of bubbling noises...   I'm
> thinking I should see a free flow again from the hose that goes to the
> pump, the outlet from the gas tank....but it is only a little trickle....Is
> that the way it's supposed to be?  Should I see a strong flow out of the
> unpluged line to the pump from the tank?  The gas cap is still off, the van
> is raised a bit on the passenger side, there is appx 1/3 tank of fuel
> aboard...
>
>    What say the fuel tank experts.?
>