Good luck, I hope you get it to work, just don't strip those reverse gears out until you are certain you have a working system.
I love to experiment too. In this case you may not just want to modify the size of the holes but also the radial location of the holes in the pipe. For example, 12 oclock at the farthest downhill position rotating downward toward 4 or 5 ocklock towards the uphill side. This may help with the partially filled pipe.