The C1391 trouble code indicates a problem with the brake system’s pressure, specifically a pressure loss due to internal leakage within the Brake Booster Assembly with Master Cylinder. This code is common in Toyota Prius vehicles and often requires careful diagnosis and repair. Toyota has issued a Technical Service Bulletin (TSB T-SB-0079-18) outlining the recommended procedure for addressing this issue.
Toyota’s Recommended Repair for C1391
The TSB directs technicians to replace both the master cylinder assembly and the Brake Booster Pump Assembly (also known as the accumulator). While the TSB doesn’t explicitly explain the rationale behind replacing both components, it provides updated part numbers for both the master assembly and the brake booster pump. This raises the question: is it necessary to replace both parts, or can the repair be limited to the faulty component?
Investigating the Need for Dual Replacement
While the master cylinder assembly is confirmed faulty in many C1391 cases, the brake booster pump often retains the ability to generate sufficient pressure. This observation, coupled with the significant cost of both components, leads to the consideration of replacing only the defective master cylinder.
Furthermore, Toyota’s diagnostic documentation for the C1391 code encourages technicians to pinpoint the specific source of the failure, suggesting that replacing both components might not always be necessary. The documentation outlines procedures for differentiating between master cylinder and brake booster pump issues.
In many instances, the brake booster pump can still achieve a 3.8V reading for accumulator pressure, often correlating with a healthy “CONDITION 2” as illustrated in Toyota’s documentation. This further supports the argument for a more targeted repair approach.
However, an extended coverage document also recommends replacing both assemblies. This seemingly contradictory recommendation suggests that Toyota likely had compelling reasons for this approach, potentially related to long-term reliability or preventing recurring issues. The increased cost for Toyota to provide both parts under warranty implies a strong justification for this dual replacement strategy.
Conclusion: Balancing Cost and Reliability when Addressing C1391
The C1391 trouble code presents a challenging dilemma: following Toyota’s recommendation ensures adherence to manufacturer guidelines and potentially mitigates future problems, but involves a significant expense. Conversely, replacing only the confirmed faulty component offers potential cost savings but carries the risk of overlooking a potential future failure in the brake booster pump.
Ultimately, the decision requires careful consideration of the specific vehicle’s condition, the diagnostic findings, and a balanced assessment of cost versus long-term reliability. Consulting with a qualified Toyota technician and referencing the official Toyota documentation are crucial steps in making an informed repair decision.
