It's the only round thing on the driver side of the tranny. Follow the vacuum line from the engine bay to the transmission. It ends at the vacuum modulator.

There's a rubber cap in the middle of the modulator. Pry it off. You'll see a bar sitting in something that looks like a castle nut. Pull the bar, which is acutally a T handle, so it clears the turret. Turn the handle as much as you think is safe then press it back in place. I think it's CW to reduce flaring. I wouldn't go more than a quarter turn at a time but that's just me. The handle pulls all the way out. Just push it back in if you pull too much.

Sixto
93 300SD