the alignment of the barbs is important, if they are horizontal,
and enough air in the res, air will be sucked into the loop.
I keep my res at 45ยบ bolted to the lateral side. works for me.
You can rotate the res so that the out is lower than the in, but that's like
duck tape.
You can keep it vertical (the reservoir) for a few days,
fill it as needed, should work as you replace the air with more water,
after that, you should be able to place it horizontal with no issues.