I have been trying to fix the brakes on my 1981 300SD and I am stumped… The problem is that I only get one stop, after that, the brake pedal gets hard and I lose power brake assist. Putting a vacuum guage on the vacuum port on the check valve, I get 15 lbs until I activate the brakes..then it falls like a rock
I have replaced the vacuum booster, rebuild the vacuum pump and replaced the hard line between the two. I have also capped off all the other outlets off the vacuum hard line so I know my problem is not a vacuum leak from the climate control, etc… its only the vacuum booster connected to the vacuum pump. I have also uncapped the oil fill to see if it would help since the vacuum pump exhausts into the crankcase…no change
Any Ideas?

It sounds like the only thing you haven't ruled out is the brake booster.

I thought he meant brake booster when writing vacuum booster. If so, then check the o-ring on the booster. I think I've read of cases where a failed o-ring can cause the booster to lose vacuum.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

You're probably correct.

brake booster filled with brake fluid. can't hold a vacuum. can be master cylinder (leaking into booster) or booster itself. In my case ('80 300sd) it was the booster. I'm still confused how it pulled the fluid out of a master cylinder that was just fine

Check valve where main line attaches to the vacuum booster, just a thought.

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"