GM O-ring Woes
Some owners of select GM vehicles may comment on a low brake pedal during braking. See the application list at the end of this bulletin.
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 for any reason. This may be caused by a small vacuum leak between the master cylinder and the brake booster. There is an O-ring between the master cylinder and the brake booster that may have fallen out during prior repairs.
Inspect for the O-ring before performing any other diagnostics. 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 off.
Applied vehicles include:
* 2009-2014 Buick Enclave, 2010-2012 LaCrosse, 2011-2014 Regal, 2012-2014 Verano and 2013-2014 Encore.
* 2009-2014 Cadillac CTS, SRX and 2013-2014 ATS, XTS.
* 2009-2012 Chevy Colorado, 2009-2014 Corvette, Impala, Malibu and Traverse, 2010-2014 Camaro and Equinox, 2011-2014 Cruze, 2012 Captiva Sport, 2012-2014 Sonic, and 2013-2014 Spark.
* 2009-2012 GMC Canyon, 2009-2014 Acadia and 2010-2014 Terrain.