My brake pedal is just a little bit harder than it should be. Is there a way to test the booster to see if it hold vacuum? I have a vacuum tester.

Not familiar with that specific model, but my w210 had hard brake pedal. The leak turned out to be the seal between the master cylinder and the booster. If you have a smoke machine, or even just a pack of cigarettes, you can smoke the exterior of the booster, and the booster vacuum line, to potentially get a visual on where it's leaking from.

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Current fleet:
1998 E300 turbodiesel
2008 E320 bluetec - leaky oil cooler seals replaced
2012 S350 bluetec
2015 GL350 bluetec

Sold:
1996 Passat TDI
1997 E300 diesel

With a hand pump you can apply vacuum and see if it holds.

Also check your system vacuum, you could have either a leak somewhere or a worn pump or a bad check valve.

There is also a one way valve that goes to the booster. Check to make sure that it will pull vacuum from the pump side but hold vacuum from the booster side.

Should the brake booster hold a vacuum indefinitely?