Now, i don't have a mark, I have a 91 Conti, but it kinda sounds like the air ride confuser (computer) senses a failure in the front (both sensors not being nearly equal in position), and goes into a fail safe mode, that causes both sides to fill untill the working side's sensor matches the other side.
Also if vent wasn't working... wouldn't rear eventually catch up to height of front? or does the Mark not drop down like my conti does when parked?
(actually venting both front and rear down to a preset level which is slightly lower than normal ride height)