I just did some emperical testing (with a 1/4 full water bottle) to see what acceleration is required to slosh the water to the upper end of a decending tank. Not much, but it is tough to keep it there. It has to be continually accelerating, not just speeding earthward, or that old devil gravity pulls the fluid forward. Pressurization doesn't change gravity, it just forces fuel already in the lines out against the force of gravity.

I've done some fairly extended "dive bombing" runs of six or seven seconds without faltering, but I can't say how full the tank was at the time. I do tend to do most of my verticals early in the flight, as I do worry about sucking air as the fuel runs low.

Once I get up I tend to loose track of time. I've sucked tanks dry dozens of times every season. Keep the nose slightly down. Regain as much upwind distance as you can if you need to and loose height with shallow spirals early on in the glide.