There can be many reasons for squeaky brakes but the two most common causes are oil or some other contaminant making its way onto the pads and rotors and uneven bedding in of brakes pads causing vibrations in the braking system.
Cleaning mountain bike brake pads.
How to determine the type of brakes.
Cleaning mountain bike disk brakes is done by removing the wheels wiping disc brakes with a clean rag and brake cleaner or isopropyl alcohol.
If using brake cleaners make sure you use bicycle specific ones as car brake cleaners sometimes include oil to stop cast iron discs from rusting james alberts sram s brakes product manager has this to say.
Squeaky disc brakes are hated far and wide in every corner of the mountain bike community.
With a small enough file and plenty of care this can be done with the brakes still on the frame use a thick cloth to protect the rim.
There can be many reasons for a squeaky brake but the two most com.
V or rim brake cleaning.
Rim brake pads can filed to assist them bedding in when new or to freshen up a well used set.
To tune the brakes up even more scuffing up both the pads and calipers with fine sandpaper and finally reinstall and bed them in.