My brakes did that after I let them get over heated on a steep hill. Completely lost brakes. I had that other symptom of them being 'stiff' too a couple times. After the worst abuse I'd put them through (downhill at Death Valley) and they cooled off enough, they seemed to go back to normal for a while. For a couple years they were fine, because I changed how I operated the brakes when heading downhill.
My master cylinder failed like a year afterwards. Pedal going all the way to the floor. Not sure which occurred first or if one precipitated the other or not.
That loss of brakes going down that steep hill in Death Valley taught me to
downshift the tranni and not to use the brakes, unless I had to, and only for short periods. That method worked great for the 12 years I owned the rig. But as I said, I did replace the MC a year after the Death Valley incident.
I didn't grow up in a truck driving family so I didn't know I was suppose to be downshifting instead of braking. I got pretty good at it after a year or so of practice.