I'm out of town.. visiting and on someone else's computer so I'm writing very quickly. I don't have my Haynes book with me, but the test I mentioned is for the Vaccuum Brake Booster.. not the vaccuum pump as I may have led you to believe. In any event if you could get your hands on a Haynes Manual then it has some steps to follow. Keep us posted.
1982 300D (266,001 mi.) looking and running great
1976 450 SLC
1982 Jaguar XJ6 (loved, but gone)