Good reply, Fred. I was thinking along the same lines about his bracing back the cover - not that safe. Just popping off and plugging the air supply temporarily is better.
In my Monterey's electric bay is a combination of valve/solenoids. Don't know which is which, but the two-part, square black and silver boxed one with two black "out" hoses likely depends on the other round silver solenoid that has one "in" and one "out" blue hose, and which is apparently "upstream" or first in line from the air tanks. I'm guessing the blue hose "T'd" in between them goes to the door seal bladder, which should have its own tranny-controlled solenoid under the right front coach corner somewhere. The two-box contraption then probably controls the opening and closing of the step cover.
Does that sound about right, or am I getting things turned around? The one black box is made by Humphrey, #D1052, 4.5 watts; but your Marquis' may be different. Couldn't see any numbers on the cylindrical solenoid, although they may be covered by its large mounting strap.
-Joel
Addendum: you posted again before I got this one in, and yes, you fellows' Marquis configuration is different than mine. I was thinking along the lines that if he removed the wrong hose, he might lose air to the door bladder(s) too, no big deal. As long as he capped it off so as to not end up with a constant air drain.