Spoke with James Triana today about this problem and thought I would pass this on. It applies to my '95 U320 as well as other years/models.
The step cover is operated by a double-acting air cylinder. This means that air is applied to one side of the piston to open the cover then to the other to close it.
There are two possible causes of the step cover closing when air pressure falls.
1. As air pressure falls the piston receives less air on one side thus forcing the cover open or closed no matter how the switch is set. The leak will be more on one side of the supply only.
2. The piston rod seal is leaking at one end with the same result.
Keith
Thanks for the info as that explains why mine opens when pressure drops below about 40 psi after being parked for a few days.