Brake pedal clunk/pop/thunk
 

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Sounds like the master cylinder

Get someone to stand outside of the car to test where the sound come from.

or get them to hold the master while you hit the brakes. also unless the brakes were recently done the pads shouldn't be sitting funny causing the clunking. I think maybe the one way valve in the vacuum line to the booster could be boned or on its way out so its not holding pressure like it should so it leaks at high pressure