Owners of a 2012 Chevy Captiva Sport may comment about a low brake pedal. During diagnostics, DTC C027B may be stored in the EBCM. This concern may have started after the master cylinder was unbolted from the brake booster for any reason.
This may be caused by a small vacuum leak between the master cylinder and the booster. There is an O-ring between the two components that may have fallen off or dislodged during the prior repairs.
Inspect for the O-ring before performing any other diagnostics for a low brake pedal issue. Replace the O-ring if missing or damaged.
Tip: Exhaust the booster vacuum by either pumping the brake pedal or pulling the check valve out prior to removing the master cylinder. The suction may be helping to dislodge the O-ring when pulling the master cylinder away from the booster.