He he, it looks tricky all right, but it's not.
The thing is, there's pressure drop, then there's relative pressure (relative to atmosphere).
You'd think that if you have a leak anywhere in a loop, that water would spurt out of it, but that's not necessarily true: the pump intake can have a relative pressure that's below atmospheric, so if you have a leak/crack there, it'll suck air in.
Electric wise, think of a battery where the negative terminal is below ground.
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