The main vacuum hose goes straight from the vacuum pump to the brake booster. It should have a nipple on it about halfway.
Check at the nipple to see if you have vacuum (should be at least 16 hg of vacuum.If not,take the hose loose at the booster (17 or 19mm wrench) and plug the end.Check vacuum with it plugged.If still no vacuum,the pump is bad.
Mercedes Benz Master Tech
ASE Master Technician
Owner of "Behind the Star Inc."