Why wouldn't you have prime when the tank is low? It's drawing from the reserve line
If you carbs are right, you shouldn't have to prime for any reason. I only use it for cold starts. Once it's running, it stays running.
The return line can work, but it's not actually a pickup. It can only pull what's in the return line. If the ski has been sitting and the line is dry, it's not going to work. If you're out riding, the lines should already be primed, you shouldn't need a primer for that, if you do, your needles are leaking or your fuel pumps are weak.
The only reason I can think of to prime a low tank is if you run it bone dry on primary before switching to reserve. Not sure why you'd keep running it on primary when it's running out of fuel. I always turn my selector to reserve during warm up to prime both lines before riding so it's ready to go when I change it.