One problem related to brake antilock control unit/module has been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
Brake pedal at times will go all the way to the floor, other times it seems like you have too keep pushing way harder than normal to stop almost like something's pushing you. Have been to dealer nine times they replaced the master cylinder twice, now they say the problem is caused by the electronic brake control module and there isn't anything they can do about it.