Usually this squeak comes from an accumulation of clutch dust on the clutch fork where it contacts the throw out bearing. I cleaned mine with a wire wheel and greased it when I had my engine out, but now it is squeaking again 10k miles or so later, it is normal, the forester does it as well. That is common for these cars. Unless you hear yours coming from inside the fire wall, go under there with some lithium grease and do work.
P.S. LOL at thinking air in the lines would cause this.