I've got a four loop (zone) hydronic heat system. Each zone is a home-run loop from the manifold. Three of the loops are on the first floor, and one is on the second (bathroom only). I've tried a bunch of things, but don't get the flow in the second story loop, even if it is the only zone opened. THere's the pump, a check valve, then the mixing valve then the switching manifold. It goes from there through each loop, back to a common return manifold. The three zones on the first floor all get warm when their valves are open, the one for the upstairs doesn't. Thoughts on how to get this loop free of air?