Brake woes

[Lou Boffa]

Gentlemen (and Ladies) I'm stumped. Me and a bunch of old mechanics can not figure out why my 70 C10 brakes (drum all around) will suddenly lock after I've been driving for a while. I can go back the next day, and she is fine-for a while, then they will lock again.
About the brake system. I added a brake booster years ago. has braked well-as well as drum brakes do--for years. Then the locking brake thing started about a 5 months ago. Had a shop work on it and we have been through replacing brake lines, several boosters and actually had the old booster rebuilt. The last suggestion was to do a disc brake conversion on the front, which isnt a bad suggestions, but I am attached to my steelies and 10" dog dish hub caps. Its my understanding that I would not be able to use my old wheels after the disc conversion.
So, all this is to say, Help! Whats the best way to get her back on the road and stopping --reasonably well?
Lou