Just bought a second hand Nikon D7000. It came with a dim viewfinder and the adjustment near it won't help. I removed the lens and placed it back, it became clear, however, it gets dimmer and dimmer after each shot.
The viewfinder in the D7000 has an LCD overlay. If the LCD isn't receiving any (or enough) power, the LCD will not be fully transparent and the viewfinder will be dark. You will want to check that the battery in the camera is good and fully charged. Given how you say it gets dimmer as time goes on, it could simply be that you're using an older battery that's losing its ability to hold charge.
Other possible causes might include a mis-seated lens (but since you already dismounted and remounted the lens this is probably not the issue); or possibly stuck aperture leaves in the lens—this can happen with older vintage lenses when age causes the lubrication of the iris leaves to turn sticky. To test this out, try using a different lens on the camera and seeing if you have the same issue.
The small wheel by the viewfinder, btw, is the diopter adjustment. It's changing the "prescription" of the lens, as it were, adjusting for near or far-sightedness, if you want to use the viewfinder without glasses. It has no effect on brightness.