Two problems related to brake master cylinder have been reported for the 2004 GMC Sierra. The most recently reported issues are listed below.
I was coming down a hill near my home & tapped on the brakes to slow down. Suddenly alarm bells started dinging, the engine warning light came on & "service brakes immediately" popped up on the message center. The brake pedal was slightly softer than normal but I made it home ok. The following day I took the truck to my mechanic to have him check out the problem. Much the same as many others have complained about, all four brake lines between the master cylinder & the abs controller were badly rusted out while one had failed completely (causing the alert/alarm). The lines from the abs controller to the individual wheels are also completely rusted but as of now haven't failed. He replaced the one ruptured line & suggested that I have the others done as soon as possible as they all look very bad. Looking at the undercarriage of the vehicle there was very little overall rust on any other components or the frame, just on the brake lines. Estimated cost for replacing all the lines as needed - $800.
Rigid brake lines from the master cylinder to the abs module rusted through causing a catastrophic failure of the hydraulic brake system.