I bought an F350 a few months ago and started noticing a distinct squeaking sound when I press the gas pedal - I mostly hear it when moving slow or being stopped, but that's likely because I can't hear it over the sound of the diesel when moving.

The pedal squeaks when being pressed down when moving but also when stopped. When I press the pedal down real hard, I hear a hissing sound (is the master cylinder leaking air?).

Any advice on what to check or if this is normal? Thanks much.