Owners of 2009-2014 Chevy Impalas and Malibus may complain about a low brake pedal. During diagnosis, a DTC C027B may be found stored in the EBCM.
This concern may have started after the master cylinder was unbolted from the brake booster during a recent service. This may be caused by a small vacuum leak between the master cylinder and booster. There is an O-ring between the two components that may have fallen off during a prior repair.
Reinstall the O-ring. If damaged, naturally, replace it.
TIP: Prior to removing a master cylinder, 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 causing the O-ring to dislodge when removing the master cylinder.