C1252

Symptoms

• ABS and brake warning lights illuminated (often with beeping on Prius)
• brake pedal feels hard or stiff
• brake booster pump heard running excessively
• loss of power brake assist — a critical brake system failure condition.

Probable causes

• 1) The root cause is almost always an internal brake fluid leak within the sealed brake booster / master cylinder assembly — the pump runs too long because the system cannot build or hold pressure. 2) Faulty pump or accumulator (accumulator pressure leak; related code C1391 'Abnormal Leak in Accumulator'). 3) Worn O-rings causing internal pressure loss. 4) Often accompanied by C1256 (low accumulator pressure): C1252 is the ECU overworking the pump
• C1256 is the resulting low pressure.

Diagnostic steps

1. Read all chassis codes and note companions such as C1256 and C1391 — they point to the internal pressure leak.
2. Key the ignition on and listen to the booster pump: continuous or abnormally long run time confirms the condition.
3. Monitor accumulator pressure sensor data with a scan tool to verify the system is losing pressure.
4. Inspect for external brake fluid leaks; if none are found, the leak is internal to the booster/master cylinder unit.
5. Replace the brake booster / master cylinder (brake actuator) assembly, which is the usual repair for an internal leak.
6. Bleed the system per the manufacturer procedure, clear codes and verify the pump cycles normally. Warn the customer not to drive the vehicle until repaired — loss of brake assist can progress to total loss of braking assist.