NV9 drain question

So I have an NV9 with 3x420mm rads, I had to build in a hurry and my drain port is in the pump/res (I know, I know...). The bottom rad is the lowest point of the build and no 3rd port on that rad sadly so I'm a bit stuck.

I know you can vacuum fill a system, but could I suck all the liquid back into the empty res then open the drain after releasing the vacuum and do that a couple of times to fully empty the loop? Or is my thinking extremely flawed? I suppose I don't need to "fully" empty the loop but you know what I mean.

Anyway, I shouldn't have rushed the build but I was taking care of someone who was injured at the time and had like 48hrs with my PC and wanted to swap the old hardware out of my old rig into the NV9 with a few upgrades.