When you get the mighty-vac check to see the booster is holding a vacuum. If its not replace the O-ring between the booster and master cylinder. If it still isn't you might have a bad booster. Check for fluid if you replace the seal, the booster should be completely dry inside.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche