I have a 1995 f250 that has a soft brake pedal. I have changed the master cylinder and no change. I am not loosing fluid, but if I remove the vaccum line to the booster the pedal is rock hard and does not fade.
Do the brakes work/ stop the truck? Most Super Duty trucks have a poor brake pedal feel. There is an older TSB for a low brake pedal and the fix was to replace the master cylinder with a larger bore. It raised the brake pedal feel, but it still "sank" if the engine is running. Ford considers the pedal sinking normal with the engine is running. If I was still unsure, I'd try to compare it to another truck. From my experience, it's a normal feel.
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