A little more information would be helpful. Did either of the two mechanics check the system for fault codes? If not, be wary of letting them try to fix your car! For Mechanic A to recommend replacing the tandem pump without first checking for fault codes doesn't make any sense, unless of course the pump is leaking. Did he check the actual pressure? Mechanic B clearly doesn't know much about the ABC system (Air cells???).

Diagnosing the ABC system can be quite complicated, but in many cases it can be very simple. MB has issued several bulletins about how to deal with the different failure scenarios common to the system. Since you say you're not having any ride height issues, I think your problem can be solved quickly and inexpensively (relatively!).

The first thing you should do is have the system checked for fault codes. I'd bet that you'll find code "C1525-System Pressure Too Low", and/or "C1526-Malfunction in Pressure Supply". If so, a simple inspection for any leaks should be performed. Even a small leak can delay the pressure build-up after starting the engine, resulting in the warning message you're getting. If no leaks are found, the cause may be as simple as the system pressure sensor not responding quickly enough. The important thing to remember is that the ABC control unit looks for pressure build-up immediately after start-up. Any delay will trigger a warning message.

Of course, there are many more possibilities. The scenarios I've described are the most common, in my experience. If you get the warning message immediately after starting the engine, you should proceed as I've said. If the warning appears while driving, that's a whole different ballgame. Have the system checked for fault codes, and let us know what you find.