[Vwdiesel] hard brakes

[Chris Jude]

a couple months ago i replaced the brake MC on the 1980 Audi 5000S because
the brake pedal would go soft while braking.  it would still get the job
done, but the pedal travel made me nervous.  Now, after just a month or two,
the pedal is pretty stiff most of the time, and it's a bit more difficult to
brake.
When i bled the brakes after the MC change, the lines were pretty gross, but
we did a pretty good job of flushing them out.
 The chilton book refers me to the brake booster for this problem, with no
further info on the brake booster.  I'm wondering if i should be looking
into a rebuild, a replacement, or somewhere else.

any input?

thanks,
chris jude
carnation, wa