Hi,
I don't hang here as much as i used to since i have no place in my current apartment for a system but it is nice to check in every now and then and see what is happening!
I have a quickfix for you that is easily checked!
Ditch the internal system, keep the horizontal line in the bottom (need to drill a few holes in it to make it work as a drain) then install the bends as shown with the red line in the pic below. this way you will have an upside down U that will empty your system when the level in the bed exceeds the level in the bend.
I have the same design on my system, only with clear hose instead of PVC pipe and it works without problem.
br,
simon