Vacuum problems when breaking, can't stop!

[DCL]

Hi,

Hope someone can help me, it's my first mercedes ('81 300SD).
(have wanted one of the old big ones since I was a kid

The vacuum pressure goes to 0 as soon as I step on the breaks.
I checked the pressure from the pump directly, it goes to about 15-16
after around 30 - 50 sec.
The system gets around 10-15, but everything drops to 0 as soon as I hit the breaks.

Do you thinks it's the Vacuum pump?
Or something with the powerbreaks?
Or am I lost here?

How can I test better?

I also get the standard problems of loosing the powerbreaks as soon as I step on them, and the engine has to be stopped manually (or wait for 1 min untill the pressure build up again.)