Bake vacuum booster leak

Listen for that hissing under the hood when the engine is running. Most likely you won't here it there. If it's loudest under the dash board, suspect internal leakage in the power booster.

GM uses some unusual, trouble-prone designs. If you get a rebuilt booster from an auto parts store, they will have a plastic cone in the front to hold a valve in position. Don't remove that cone until you're ready to bolt on the master cylinder. If you get a used booster from the salvage yard, once it's removed from the car, always hold it in that same orientation. Don't lay it down to haul it home. If they bring you one that was already removed and has been sitting on a shelf, ask to have one removed from a different car in the yard. Better yet, find a yard where you pay your buck, then take your own tools to remove parts. That way you'll know it stays in the upright position.