Hit Man,
If you want air out your center vent, I inadvertantly figured out a way to do it. Just take out your control unit, and look back in the cavity and to the right. You'll see a pink pod with a vac line coming out the front. Pull the line, and plug it up with a screw. Then pry the pink cap off (it's tricky, because there are 4 pins you have to try and hold in at the same time. Or you can just break them off). A spring will fall out. Pull out the cup with the rubber diaphram (it's attatched at the back with a lever). Now you should have center vent air, which can be controlled by the slider at the vent!

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head