Great info, thanks.:bigthumbsupThe pump that I got with my kit can move a lot of water, but is really bad at self-priming, so it can't pull air worth a damn.
When I tested the pump, I held the intake end in a bucket of coolant and it wouldn't pull 1" of air to prime. I had to tip the bucket almost horizontal before it would pull the last 1/2 - 1/4" of air and start pumping.
When the line drops below the level of the pump and air gets trapped just in front of the inlet of the pump, the water below the pump was not allowing the air to escape, and the water above it was not enough weight to push the air out, and since the pump can't pump air worth a damn, the air just sits there and the pump doesn't prime.
By moving the hose so that any air will just bubble up the hose to the reservoir, this is no longer an issue and you always have prime. I drew a couple crude pics real quick to help explain whats happening. In pic 1, the air is stuck at the inlet with no where to go.. if you loosen the intake clamp it will escape and prime is restored, but that is a high point and air will collect there. In fig 2, you remove that high spot and now the reservoir is the high spot, so no air collects.
I don't know if any of these coolant pumps are any good at pumping air, but I know this bosche brand that I got is not.