Shane, were you able to measure vacuum at the top of the problem line? I assume that you get good natural vacuum on your 8% slope. Maybe the long flat 3/16 tubing at the end reduces it. If so, could you put the collection tank at the bottom of the steep part, and pump it out through a transfer line when full. Seems like that way you get the advantage of natural vacuum but not the downside of the flat run. Just a thought. Might be an interesting test.
Dave