560SL Brake Booster Issue
 

I have lost brake assist from the booster and therefore looking for some troubleshooting help. When pressing the brake pedal I am hearing a large air leak inside the car from the brake pedal area under the dash. I have searched the forums and see that some people have fixed booster issues by replacing the vacuum hose and check valve assembly, located under the hood between the intake and booster. Whilst my check valve does not look good, and is probably the original, the air leak noise I am getting is heard only inside the vehicle and not under the hood where the check valve is. Because of the obvious air leak noise inside the vehicle I am thinking replace the booster or could it just be a seal maybe? Any ideas before I start buying parts?

Sounds like the diaphragm inside the booster has failed.