While this car isn't a MB diesel, it's a Peugeot but the brakes should be similar. I haven't driven this car in over 3 years and all the brake fluid has leaked out of it. After starting the car and stepping on the brake, the pedal goes to the floor but won't come back up when I take my foot off, even if I try to pull it up by hand. I was going to add fluid and bleed the system but that's hard to do if the pedal won't come up. What is causing the pedal to stay down? I assume it's vaccum but why won't it release? Do I need a new brake booster or is there a check valve somewhere that is bad and allowing vacuum to constantly keep the pedal down? Is there a big spring in the booster that may have broke preventing the pedal from returning up? It has a conventional vacuum booster and mechanical vacuum pump.