Chassis, generic

C05B1 Brake Hydraulic Circuit D Leak

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

C05B1 is the leak code for hydraulic circuit D. The only public maker use is GM's hybrid brake controller of 2016–2019, where it covers the rear axle circuit. Two checks feed it: during pressure hold phases with the pump off and the car at rest, a calculated rear leakage above 0.3 ml/s, judged over more than 300 s of cumulative braking; and, with more than 5,000 kPa at the front axle and a steady pedal for 3 s, a master cylinder pressure lower than the rear axle pressure. Type A, MIL after one trip. Its partner C05B0 covered the front axle there. A Continental patent shows another approach: build pressure with a motor-driven plunger and judge tightness from the slope of the pressure decay. FMVSS 135 demands a red warning on major fluid loss. Inspect lines, rear calipers and the hydraulic unit.

Likely causes

Fluid leak at a rear brake line, hose or caliper
Internal leakage of an inlet or outlet valve in the hydraulic unit
Air in the rear circuit after service

Symptoms

  • Brake and ABS warning lamps on, MIL possible
  • Longer pedal travel and weaker rear braking

Repair estimate

DifficultyHardWorkshop job
Parts and labour€50–1,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

brake circuit dabs hydraulic unit

Related codes

C05B0C055DC055EC0049

References: gsi.ext.gm.com, gsi.ext.gm.com, patents.google.com, law.cornell.edu.