Skip, FWIW.

Mercedes diesels use a pneumatic vacuum system to run things like door
locks, etc. (at least on my wife's baby, an '82 240 D). The driver's side
valve will lock and unlock the doors when operating the lock post or the
key. It's perfect for an alternate air valve, weighs maybe an ounce, is
about 2 1/2 inches long by maybe 3/8 in. dia. and costs a pittance - don't
remember what I payed for mine but it was just a few dollars.

The valve utilizes an O-ring to effect the seal and the seal works quite
well. The Mercedes system utilizes an accumulator so you can lock and unlock
the doors without the engine running and you can come back to the car weeks
after it was last run and the doors will unlock, so the seal is really good.

Dan Schaefer