F,
As it is set up in your pics, the Main Drain is almost fully open... If you are trying to "feel" suction at the Main Drain covers, you are just wasting your time.. It is almost impossible to feel any suction right at the main drain. Turning the valve so that the handle is pointed at the word "Skimmer" on the pipe, will close the Main Drain.
"Most" skimmers only suck in water and have no return water path. There are some venturi skimmers, but they are not used all that often.. I would not think that a pool builder who'd use ball valves and a single speed pump, would be using a fancy Venturi skimmer..
Show us some pics of your skimmer.. At least one with the skimmer basket removed so we can see down inside..
Thanks,
Jim R.