This happened to me as well, I changed the MC out about a year ago I believe, and noticed the booster had fluid in it. The brakes had always been a bit hard and under-boosted...and now I know the reason. As it is now, when I put my foot on the brake and start the car, the brake pedal does not move downwards noticeably... so the booster isnt doing much for me. If I do one medium-hard stomp on the brake and then try to do another one I will not have any vacuum assist. I have just learned to live with it for now, as boosters are expensive, plus the car brakes very very well if you give it enough muscle. I saw a site that had remanufactured ones for like $100 but can't remember where it was.

__________________
1985 CA 300D Turbo , 213K mi