What was found
AIRCRAFT WAS GROUNDED: DURING TAXI LEFT BRAKE PEDAL MAKE A HIGH PITCH SQUEAL WHEN PEDALDEPRESSED. YOU CAN FEEL A VIBRATION IN BRAKE PEDAL AND SEE ACCUMULATOR PRESSURE FLUCTUATE SLIGHTLY. FLIGHT ATTENDANTS COULD ALSO HERE THE NOISE IN THE CABIN. MAINTENANCE FOUND THE HYDRAULIC RESERVOIR AIR PRESSURE WAS LOW, SERVICED RESERVOIR AND BLEED THE BRAKE SYSTEM PER AMM 12-12-00 AND 32-44-00. A SATISFACTORY OPERATIONAL / TAXI CHECK WAS ACCOMPLSIHED WITH NO VIBRATION NOTED IN THE PEDALS.