I had the same a couple years ago on the old GMC M135 at work. Another unit had done the brakes previously, and the slightest touch (anything more than two toes worth of pressure) would just about send you through the windshield. There was also the feeling that they were sticking. The wheel cylinders were about 8 years old at that point, and I did find that one or two of them were starting to leak. As well, whomever did the brake cleanup used a disc sander to take the ridges off the inside edge of the drums, leaving about an inch of drum in poor shape. Lastly, they had used the master cylinder off an M35 MLVW, which, while it looks the same as an M135, has a small residual check valve inside.
Rather than trying to repair faulty work of others, I went clean with 12 new wheel cylinder, 6 new drums, and new shoes all around. I also pulled apart the master cylinder and removed the check valve. Part of my motivation to put all new was also to take advantage of the parts still being available in the supply system for the 1-1/2 ton trailers.
Long story short, on completion the brakes worked out great. They feel exactly as they should, and hopefully will remain trouble free for another decade, at which time they will likely be the problem of someone else. The truck also had an airpac from an M35 installed.....I left that alone.
So which exact cause was resulting in the brakes locking on I cannot say. All I can say is that by doing it by the book (or as close as one can considering what parts are available), the problems disappeared.
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