Brake peddle goes to floor on 1978 300sd Mercedes when engine running. Have replaced master cylinder twice, checked for leeks, tested booster and it holds vacuum, vacuum pumps at 25inHg, checked for fluid leeks and found non.

Check rear brakes for wheel cylinder leak. Bleed the entire system, starting with the rear brakes and then the front brakes.

you can also try bleeding the master by it's self, then the system like befree said.

If you've tried all other recommendations and still have problem try this.

brake lines on these cars sometimes run through the "frame," if the leak is inside of the "frame" area it may be hard to see. Recheck lines carefully.

Also, some Mercedes require a brake push rod adjusment when changing the master cylinder. Will check to see if yours does.