C1391

Symptoms

• ABS
• brake system and traction control warning lights illuminated
• soft or spongy brake pedal requiring increased pressure
• brake actuator pump motor running abnormally often (every few seconds with the vehicle in Ready state and pedal released) or continuously
• hissing/buzzing noise from the brake actuator
• intermittent braking performance
• potential loss of braking assist.

Probable causes

1) Internal leak in the brake booster / master cylinder assembly due to degraded internal seals (documented common failure on 2012-2016 Prius C and other Toyota hybrids). 2) Faulty accumulator losing pressure. 3) Leaking hydraulic unit / ABS actuator with improper internal sealing. 4) Malfunctioning ABS pump. 5) External brake fluid leak in lines or components.

Diagnostic steps

1. Read ABS codes with a capable scan tool (Toyota Techstream or equivalent) and confirm C1391 is current.
2. Perform a visual inspection of the accumulator, brake lines and ABS components for visible leaks or damage.
3. Listen for the brake pump: internal leakage is suspected if the pump motor runs continuously or cycles every few seconds with the pedal released.
4. With the scan tool, monitor the accumulator pressure sensor while commanding the pump — a rapid pressure drop with the pump off confirms an internal leak.
5. Perform pressure tests on the accumulator to pinpoint the leak source.
6. Replace the failed brake booster/master cylinder (and accumulator) assembly with OEM parts, then perform the full system calibration/bleed procedure and verify brake operation.
7. Check for Toyota extended warranty coverage — Toyota has acknowledged this failure on certain models.