Possible/most likely the signal light switch, which is part of the multifunction switch, even though the signal lights work ok! Brake light circuit passes through this switch before going to the brake lights! Do you have battery voltage on both sides of the brake light switch with pedal depressed?
Does your high mounted (Cyclops) brake light work, it's wired straight from the switch. The rear (tail)/brake lights go through the multifunction switch, (turn signal switch). Fuse #13 in the panel fuse box.