Chassis, generic

C055D Brake Hydraulic Circuit B Leak

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Passenger-car brakes are split into two hydraulic circuits so that one can still stop the car if the other fails. Brake-by-wire units with a motor-driven plunger can estimate fluid loss directly: they compare the plunger travel with the measured pressure. In GM's 2025 EBCM tables C055D is the leak check of one circuit (listed as circuit 1): if the calculated leakage passes 500 mm³/s (a second stage 2000 mm³/s) for 0.1–0.5 s while braking with the circuits separated, a Type A fault with MIL is set after one trip. C055E covers the other circuit. FMVSS 135 requires a red brake warning when a split system suffers a major drop in fluid level or pressure. The car then brakes with reduced effect and longer pedal travel; it must be checked before further driving.

Likely causes

Leaking brake line, hose, fitting or caliper in this circuit
Worn seal in the master cylinder or the brake-by-wire unit
Air left in the circuit after service, read as compliance or leakage
Overly sensitive leak calculation in the brake module software

Symptoms

  • Red brake warning and amber ABS or stability lamp
  • Soft or sinking brake pedal, longer stopping distance
  • Falling fluid level or wet spots at a wheel

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–2,500
Time0.5–4 h

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

Parts involved

brake circuit babs hydraulic unit

Related codes

C055EC055FC05B0C05B1

References: gsi.ext.gm.com, law.cornell.edu, static.nhtsa.gov, bosch-mobility.com.