Pull the two fasteners that hold the master cylinder to the booster and check for fluid in the booster.

Brake fluid will ruin a booster, so if you find some, flush it out asap!

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles