Two problems related to brake master cylinder have been reported for the 2001 GMC Sierra. The most recently reported issues are listed below.
I was traveling on I-95 south in virginia on the hov lanes at about 1:30 pm approaching the route 1 exit. Traffic was dead stopped in front of a moderate flow of traffic and everyone quickly slammed the brakes (including me) to slow and stop to prevent rear ending the stopped cars. Traffic was flowing at approximately 75 mph when I slammed my brakes. The brakes grabbed momentarily and then I heard a loud pop and the brake pedal went to the floor; the vehicle brakes were gone. No pressure was at the pedal even as I pumped it. The only way out was to hit the should and berm and take the route 1 exit to coast to a stop; which I did. No fluid loss was evident at the master cylinder reservoir and I couldn't see evidence of a blown line or brake caliper until I got the vehicle home and noticed the metal (steel) brake line between the master cylinder and the anti-lock brake control valve/module was leaking at a pit in the line caused by rust. The point was under the driver side floor pan on the outside of the frame and parallel with the frame. It blew a rusted pitted spot on the line under pressure.
Metal brake lines rusted causing brakes to fail and replacement of master cylinder and brake lines.