Two problems related to brake master cylinder have been reported for the 1999 GMC Sierra 2500. The most recently reported issues are listed below.
Went up to store and found out brakes were not working, pedal going to floor. Had to avoid wrecking. Hauled it home. Purchased and put in new master cylinder. Went to bleed the first brake and found a rusted break line over driver side rear. Could hear it sucking air when I was using the pump to bleed out old fluid. Glad the traffic wasn't busy.
Driving around town and wnet to slow down to stop and the brake line to the right ( passenger ) front caliper steel line blew fluid all over truck and pumped brakes until the master cylinder was almost empty . Truck stopped after this and turning into a grassy field. This truck needs all the line replaced!!! I bought the truck new 32,00. 00 and it has dollar store brake lines!! safe!!!.