Appreciate the "Ask Me" and my compliments, scary driving one of those in our roads.
Might you have to check the Brake Booster. Normally, there would be a vacuum line to the booster from the intake manifold. Common design calls for a PCV type valve at the entrance to the booster. Pls check this. It also operates on the same principle and therefore checks the same, you can blow one way but not the other; would have a sliding ball inside that acts as the valve. In some instances, it could be the diaphragm inside that might have a tear.
Of course, the Proportioning valve system must likewise be checked. I have heard a lot of brake problems with K2500 especially the rear drum.
Incidentally, there was a recall order of some sort. TSB 99-05-24-001A SEP 99 which addressed brake pull/wear, the recall involved replacing the proportioning valve.
- The Technical Service Bulletin referenced previously discusses the
issue surrounding the proportioning valve. Also, there's a corrosion
issue which has resulted in the recall of the trucks in certain states.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
Good luck and kind regards. Thank you for using FixYa.