I appreciate the responses everyone!
Regarding the air filling nipple by the left front fender and then "locking" the suspension - do I need a special pump to do this, or just a longer time with a standard tire pump?
Also, how do I "lock" the suspension?
Let me help you out just a little, since my brother and I have 30 years experience on w112 cars.
Above suggestions will work, IF YOUR SYSTEM WOULD NOT LEAK IN THE FIRST PLACE.
However, it may have dropped, because the system is not locked.
Every time any part of the car lifts or drops (front or back), the valves open (2 in front and one in the back) to add air pressure to 'level' the car again.
Since the pressure in your tank has dropped it does the opposite now.
Mind you, that you need about 150 psi to raise the car completely.
The rear will lift with less, but you need the full pressure to raise the front axle (completely).
The filler valve (just like a tire valve stem) is in the above described location, but you need really small hands and a flexible wrist to get the hose bit there to fill it, OR IF THE CAR IS UP ALREADY (yes, I know what you're thinking).
Leaving it in this state (on the floor) will stretch or break aged air bellows, maybe damage the leveling valves.
The system is generally and in large part regulated by the main valve on the left front inside fender (midway), near the top edge (where the lever in the car connects to by means of a semi-rigid cable) and which locks the system.
To lift/jack up
First of all the car should not be 'flat' on the ground, there should be a couple of inches clearance, unless all your axle mounts and bushings are worn out.
Even the engine mounts.
Be careful, because most of the time, the engine is now sitting right on the oil pan, and that's not good.
But, you have to start somewhere.
IF the car has no rust your selection is bigger than when it's not.
No rust :
at and solid jack lift points, (but be REALLY SURE of this !)
lift the car with its factory jack, to get enough clearance.
: lift the front of car on the cross member (behind the bumper, under the radiator) with a floor jack.
If the cross member is brittle use the longitudinal frame rails just behind where it attaches to.
Get enough clearance , block it (with jack stands under the frame rails)
and go again.
This will take a while if you don't want to damage the car.
There is ways to 'block' the car even without pressure in the system.
Ask again,if you need help with that.