A squeak on the brakes that goes with slight pressure applied to the pedal is more than likely nothing more than the pads. Take them out, apply copper grease to the backs and pop back in. At the very worse, maybe a new set of pads.....?
As said above, sounds like they took you for a ride changing the master cylinder. Also a lot of brake systems have anti squeal shims behind the pads which can lose effectiveness over time.
i had both sets of front pads out when they started squeaking, greased up and replaced, still the same squeak
i'll check to see if it has been replaced