are you losing any brake fluid? normally when a master cylinder leaks internally, the pedal gets soft ~~~

tried the stethoscope under the hood on the brake booster itself? there's a large spring inside the booster but unless it breaks there's nothing it can rub against

you can disconnect the pedal from the booster rod and then try the pedal

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1989 300ce smoke silver / brazil, in a constant state of flux ~~~