had similar problem on mine.. once it was the vaccuum boost and the next time it turned out to be the calipers.. right front was/had been leaking terribly.. calipers are not expensive to replace.
A way to check the vacuum boost is to turn the key once to the right, pump your brake pedal 3 or 4 times.. holding down on the last pump, then crank the car. If the brake pedal goes down toward the floorboard after car is cranked, then it is NOT the vaccuum boost. If it does not go down any further.. well, then have a reputable guy look at it. Good Luck!
1982 300D (266,001 mi.) looking and running great
1976 450 SLC
1982 Jaguar XJ6 (loved, but gone)