I think what happens is the brake booster diaphragm leaks internally and can’t hold vacuum pressure and you can hear a hissing or whooping sound. one way I knew my brake booster was leaking was to see if the brake pedal feel was better if I pressed the brake pedal while building vacuum pressure by revving the engine. What happened was that during the second or two the revs were going up (negative pressure) the brakes felt normal. as soon as the rpm went back down or stayed consistent, the brake booster lost vacuum and brakes became mushy.
BUT.. that was with a gas motor. I’m not too familiar with how diesels make vacuum.
