Owners of a 2008-2012 Cadillac CTS may complain about a hard brake pedal during a cold start.
Check the operation of the auxiliary vacuum pump. If the pump is inoperative, check the BRK VAC PUMP fuse in the UBEC. If the fuse is open, check the wiring from the underhood fuse block to the vacuum pump.
If the wiring is OK, replace the auxiliary vacuum pump and fuse. Be sure to check the black O-ring in the Power Booster Vacuum Hose adapter for any nicks or tears. If the O-ring is damaged, replace it, making sure that when installing the new hose to make a straight connection onto the pump, not on an angle, which can damage the O-ring.
The O-ring is serviced by replacing the power booster vacuum hose.