When you get that low that you have to go to reserve, then reserve and prime pull from the same remainder amount.   They are equivalent.

Some petcocks won't pull well from reserve but will pull OK from prime.  Mine is ass backwards to that, it won't do much of anything on prime but will limp along after a fashion on reserve.  Not well, just barely limping along.

That's why most of us plan to yank our fancy suction-actuated petcocks and go to a simple good flowing mechanical valve.

..... simplicity works.  

I remembered to turn a manual petcock off for a lot of years, I think I can remember to flip it off then on again in the future.