Engine speeds up when brake is pushed

You don't state the car year or model (72?). A photo of your booster would help. Since you mention a clamp, it sounds like the same basic model on my 65 Newport (Kelsey, I recall). Don't fear taking it apart, it is very simple inside. My diaphragm had a hole where the vacuum port enters (gas vapors). I couldn't find a rebuild kit or diaphragm anywhere, so cut my own out of neoprene rubber and secured the inner edge in the welded plates w/ flowable silicone. It works when tested w/ manifold vacuum, but I haven't installed (replaced w/ one from a 74 Dart, drilled new holes).

There is one rebuilder recommended on hot rod sites - "Booster Dewey" I recall. I probably have the link at home.