Trackhead you have a picture of this?? Not sure what you mean by reversed? How can that thing be reversed? I am having similar problems sucking water out?? Trying to understand this?? LOL help a guy out!

 

Notice the tab on the reservoir lid. In my case, it was facing rearward, so the flow from the radiator to the reservoir through this cap was reversed. I flipped the cap so the tab is facing forward, now the air passes through the cap in the proper direction and does not cause a syphon.



To test, take the vent tube off at the radiator. Carefully blow into this hose with reservoir half full or so. If it is set backwards, coolant will immediately flow out the short vent tubing, out of the reservoir. This is because the air is just passing over the tube that goes down into the reservoir, creating a syphon, or a suction from negative pressure. If it is set up right, it should blow air INTO the reservoir like you are blowing on a straw in a glass of water. Just don't suck in any ethylene glycol, or you'll end up in the ER on a Fomepizole infusion

I never would have thought the cap on the reservoir had a specific function, but low and behold, it does. I can see where someone might easily reverse the cap during a radiator flush or whatever else.

 

Glad it was not your head. Here's a pic that shows proper placement of the cap. Not our engine but you get the point. Good eye brah.