C1391: How to Diagnose & Fix — Abnormal Leak of Accumulator Pressure (manufacturer-specific)
4 vehicles · 1 makes · $1,200-$3,500 repair
How to diagnose and fix C1391: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Toyota 4Runner · 2003–2009
Multiple 4th-generation 4Runner owners report the brake pedal becoming hard, requiring excessive effort to stop, often accompanied by ABS/VSC/TRAC warning lights and a buzzing or continuously running brake actuator. Toyota issued service information for brake booster pump/master cylinder concerns on related platforms, and owner complaints frequently describe internal accumulator or actuator failure rather than simple pad/rotor wear. Repairs are expensive because the booster/master/actuator assembly is typically replaced as a unit.
Common Symptoms
- Hard brake pedal
- Longer stopping distance
- ABS light on
- VSC TRAC light on
- Brake warning light on
- Buzzing or frequently running brake pump
- Intermittent loss of brake assist
How to Fix
Diagnose by scanning the ABS/VSC system for stored brake actuator codes, checking whether the pump runs excessively, and verifying loss of assist with the engine running. Inspect for internal hydraulic leakage and confirm whether the accumulator pressure will not hold. Repair usually involves replacing the brake booster assembly and/or master cylinder actuator unit, followed by brake bleeding with a scan tool-capable procedure and calibration of the brake system.
Owner tips & cautions
- TipScan the ABS/VSC module before replacing parts; many owners initially chase wheel speed sensors when the real fault is the booster/actuator assembly.
- TipIf the pump runs every few seconds with the key on, do not ignore it; owners report the condition usually worsens into warning lights and reduced assist.
Sources for this issue (2)
Toyota 4Runner · 2003–2009
On 4th-gen 4Runners the integrated ABS actuator/brake master cylinder assembly (with electric pump motor and accumulator) fails internally. The pump motor or accumulator wears out so the pump never reaches pressure and runs continuously; owners get simultaneous ABS, BRAKE, VSC and TRAC warning lights, a loud continuous in-cabin alarm, electronic noise when braking, and a hard or sinking pedal with reduced assist. It is a safety-relevant failure and the assembly is one of the most expensive non-drivetrain parts on the truck. This is distinct from the brake booster/master-cylinder internal-leak issue: this is the ABS hydraulic actuator/accumulator unit.
Common Symptoms
- ABS, BRAKE, VSC and TRAC lights on together
- Loud continuous in-cabin warning alarm
- Brake pump motor runs constantly / electronic noise
- Hard or sinking brake pedal, reduced assist
- C-series ABS/brake control codes
How to Fix
First scan and inspect the wheel-speed (ABS) sensors and connectors, as dirty/damaged sensors mimic the symptoms cheaply. If the fault is the actuator/accumulator (continuous pump run, pressure faults), replace the integrated ABS actuator with master cylinder assembly - OEM 47050-60080/47050-60081 (4th gen) - and bleed/initialize the system with a capable scan tool. The part alone runs roughly $1,700-$2,300; remanufactured units are available cheaper.
Sources for this issue (2)
Toyota Prius · 2010–2015
The electro-hydraulic brake booster/accumulator assembly on third-generation Prius develops an internal fluid leak, so the accumulator can no longer hold pressure. The pump then runs constantly to keep pressure up (loud buzzing/grinding from under the driver's footwell) and eventually the system sets C1391 (often with C1252, C1253 or C1256), turns on the brake, ABS and hybrid warning lights, and falls back to unassisted braking with a hard, long pedal and much longer stopping distances. Toyota acknowledged the failure with Customer Support Program ZJB covering 2010-2015 Prius and Prius PHV brake booster and brake booster pump assemblies (10 years/150,000 miles; primary coverage has now expired).
Common Symptoms
- Continuous buzzing or grinding from the brake actuator, sometimes with the car off
- Hard, long brake pedal with little assist
- Brake, ABS, VSC and hybrid system warning lights on
- Longer stopping distances
- Brake fluid level dropping with no visible external leak
How to Fix
Diagnose with a scan tool that reads chassis codes; a running/buzzing pump with the car off and C1391 stored confirms an accumulator leak. The repair is replacement of the brake booster with accumulator pump assembly (frequently the pump assembly as well) followed by the Techstream-guided brake bleed/linear-solenoid initialization, which is required or the pedal will remain wrong. Check for remaining ZJB coverage or a goodwill claim first; used assemblies are a gamble because the same seals age on every unit.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Toyota 47070-47060 brake-booster pump with accumulator
replace the diagnosed leaking/failed pump-accumulator branch before professional Techstream bleed and initialization Fitment/availability: 2010-2015 Prius liftback and 2012-2015 Prius Plug-in 2ZR-FXE; not the Prius v branch and VIN confi…
- Toyota 47070-12020 brake-booster pump with accumulator
replace the diagnosed Prius v pump-accumulator branch before professional Techstream bleed and initialization Fitment/availability: 2012-2017 Prius v 1.8L; use for the issue's 2012-2015 Prius v branch only and confirm VIN Fitment/availab…
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Sources for this issue (3)
Toyota Camry · 2012–2014
Toyota Highlander · 2006–2013
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C1391 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does C1391 mean?
C1391 stands for "Abnormal Leak of Accumulator Pressure (manufacturer-specific)." C1391 is a manufacturer-specific brake code, documented mainly on Toyota and Lexus (often hybrid) models, indicating the brake control system detected an abnormal leak or loss of accumulator pressure in the hydraulic ABS/brake actuator. The accumulator stores pressurized brake fluid the system uses for power-assisted and ABS braking, and degraded internal seals can let it bleed down. Because this directly affects braking assist and can increase stopping distance, it is a serious fault that should be repaired promptly by a qualified shop.
What are the most common causes of C1391?
The most common causes of C1391 are: Failing brake booster / master cylinder accumulator seals, Worn or leaking brake actuator assembly, Internal hydraulic leak in the ABS/brake actuator, Low brake fluid level, Aged or contaminated brake fluid, Faulty pump or pressure sensor within the actuator, Faulty brake control (skid control) ECU. The specific cause varies by vehicle.
How much does it cost to fix C1391?
Repair costs for C1391 range from $1,200 to $3,500, depending on the vehicle and root cause.
Which vehicles are affected by C1391?
Au7o has documented C1391 across 4 vehicle models from 1 manufacturers: Toyota.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.