2024-2026 BMW 3/4 Series Brake Fluid Leak TSB

BMW has released Service Information Bulletin (SIB) 34 09 25, a revision of SI B34 09 25 dated July 2025, to address an internal leakage issue with the brake fluid expansion tank. This service action applies to various 2024-2026 BMW models, including the G20 (3 Series Sedan), G80 (M3 Sedan), G22 (4 Series Coupe), G26 (4 Series Gran Coupe), G23 (4 Series Convertible), G82 (M4 Coupe), G83 (M4 Convertible), and G87 (M2 Coupe). The bulletin outlines the cause of the problem and the necessary repair procedure to resolve the potential BMW brake fluid leak.

Symptoms

Owners may experience an internal leakage within the brake fluid expansion tank, also known as the brake fluid reservoir. In some cases, this internal BMW brake fluid leak can lead to a malfunction of the filling level sensor over an extended period. While not explicitly stated as a dashboard warning, a malfunctioning sensor could lead to incorrect fluid level readings.

Cause

The root cause of this issue has been identified as a manufacturing defect within the brake fluid expansion tank itself.

Important Warnings for Technicians

BMW emphasizes strict safety protocols when performing this service action, especially concerning high-voltage (HV) systems on hybrid and electric vehicles. The bulletin states:

• "Only properly trained personnel, who passed all applicable HV Technical Training Courses, should perform repairs which require disconnecting, or removal of High Voltage battery components on any Hybrid or Electric Vehicle. Work performed on High Voltage systems by unqualified persons may result in severe injury or damage to the vehicle. Additional safety information is found in Repair Instruction 61 00... 'Observe safety instructions when handling electric vehicles'."
• Prior to disconnecting or removing any HV component, the HV system must be disabled and secured by a properly trained HV technician with a minimum HV Qualification level. Specific training courses mentioned include "ST2324 High Voltage Drivetrain Systems" and "ST1824 Alternative Drive Part 1".
• For Generation 4 vehicles, once the HV system is disabled (indicated by the "Blitz" - lightning bolt icon in the instrument cluster), a non-HV certified technician may remove HV components, except for the High Voltage Battery.
• For Generation 5 vehicles, specific vehicle training is required, and non-HV certified technicians are not allowed to work on the high voltage system.
• High Voltage Battery removal and rework can ONLY be performed by a High-voltage Certified Technician with specific HV Battery Certification levels (e.g., ST2006 – SP44 HV Battery, ST1825 – Alternative Drive Part 2, ST2205 Generation 5 High-voltage class, ST2325 for High Voltage Battery Systems).

Repair Procedure Summary

The correction for the internal BMW brake fluid leak involves replacing the brake fluid expansion tank of the Integrated Brake system. Technicians are directed to follow Repair Instruction REP 34 51 110, titled “Remove and install/replace expansion tank for integrated braking system.” The repair is identified by code 0034200300, labeled as "G2x G8x Replacing brake fluid in expansion tank."

BMW notes that "THIS REPAIR IS MOBILE FRIENDLY" and "THIS REPAIR IS REMOTE SOFTWARE UPGRADE (RSU) FRIENDLY." This procedure should be performed on all affected vehicles before customer delivery. If a customer has already taken delivery, the repair should be completed the next time the vehicle is in the shop, provided the campaign status is