Reposted:
Booster does not assist the brakes. Car does stop but it takes a BIG leg effort.
I have 25" Hg on the large vacuum line to the booster, pulsating 14-16" on the small line from the vacuum pump, booster holds 10" vacuum for 10 minutes (may hold more but evacuating a booster with a mini vac is a bit of a job).
I plugged all other vacuum lines: ALDA, trany, locks, ign. switch, HVAC and no change.
I rebuilt the master cyl. the other day and removed 1qt. of brake fluid from the booster. Flushed and bled the brake lines and kept the reservoir tapped off during bleeding.
The only thing left that I have not checked is the check valve in the large vacuum line to the booster.
Advice, alcohol, drugs, shock therapy .... is welcome.

If the booster got brake fluid in it then it could be failing. Check all lines to the booster and all check valves to make absolutely sure you have enough vac to the booster itself, if other stuff is leaking it could weaken the booster's ability to function properly. If all checks out and its still not working, I'd guess the booster has failed due to the brake fluid getting inside.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

fails a catostrophic death and so take a look at and follow these links which explain how the booster is put together and how it works:
http://auto.howstuffworks.com/power-brake1.htm
http://auto.howstuffworks.com/power-brake2.htm... etc
The above has good graphic illustrations to help in understanding. The article even talks about how diesels have a separate vacuum pump. The booster costs over $300 so I would first concentrate on ivestigating the check valve where the vacuum line enters the booster shell, and the "O-ring" and "Pinion" seals that are located at where the master cylinder bolts onto the booster... where the rod protudes from the booster.
Good luck,
Sam

If the o ring between MC and booster was faulty, then how could the booster hold vacuum when testing through the vacuum port?
Can the booster diaphragm be shot and booster still hold vacuum?

from all you have given, I think the booster reservoir is NOT holding vacuum properly and what I don't understand is how you have tested this. Have you taken the large pump-to-booster line off and checked the in-line check valve by blowing/sucking on the check valve. Take care... what with all the brake fluid contamination you found. I'm betting that your check valve is not holding properly or there is a leak elsewhere. That's why I asked about the "O-ring" and "pinion" seals. Yes, you are correct that it's very difficult to test the large capacity booster reservoir using a small hand pump. No, I don't think the booster diaphragm can be "shot" and the booster hold vacuum... I'm not sure you have proven it is receiving and holding [ or NOT holding] a vacuum. Take that large, main, vacuum line off and check out the check valve by " huff-N-puff " if you have not already done so... for it could have failed to hold or the harder to believe failed to allow air to pass and allow air to be evacuated from the booster reservoir.

Either way, if the check valve is bad, I think you will end up replacing the entire line and in-line check valve [ ~$35-45.00 ]
Sam