Quite a lot of you newer visitors are COMPLETELY OBLIVIOUS to this hint I left in an alternate discussion post, so now it gets its own specifically-titled post so you couldn't possibly be confused. If you still are confused then GLaD help you.

http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=692212103

And to reiterate;
The switch is a DIAL. Not a lever, not a button, not a switch, but a DIAL.
If any of you played Half-Life 2, you should recognize the model from Ravenholm.

It doesn't even look anything like a lever! How do you people keep calling it one?!

And just to make sure you know, here's a provided screenshot courtesy of the Valve Developer Community Wiki;
http://combineoverwiki.net/images/6/6e/Gas_Tank.jpg
It's the thing in the top-left corner of the image.