Engine running due to stall converter???

During the warm up period,with the stat closed,some of the water is going round and round inside the pump,via the bypass hose, with some percentage looping thru the block. I couldn't speculate as to how much goes where. During the early part of this activity , there is little to no pressure yet in the system.

Vane type pumps are pretty good pushers, but they have very little ability to pull, hence why a well-pump has to be primed on a drystart. If the footvalve fails, it is often very difficult or impossible to get the pump working again..

You can't pick up air either, but you can still "suck" it thru a straw;but I understand what you're saying. I have seen that hose flat as can be when the water in the rad gelled up on a too-cold for the freeze-protection level.