A few things:

On this list, there are some that equate SBC with the Hindenburg mooring in Lakehurst NJ. . . . they would rather apply blind emotion over data and logic.

You need to get a hold of a mercedes trouble shooting manual that lists the specific code you have.

I'd think that, applying standard hydraulic theory, pre tensioning pressure would be gas pressure in the accumulator, this won't show up on a scan tool. It might be inferred by looking at pressure rise over time. A rapid pressure rise would infer a accumulator that has low gas pressure. A Mercedes manual would give more info.

Look at the links / diagrams in my post 26 , pressure reservoir would be part 3
http://www.peachparts.com/shopforum/diesel-discussion/374264-sbc-2005-e320-cdi-2.html#post3555288