Search the forum, this has been covered in much greater detail...
If cracking a fluid bleeder solves the problem temporarily, no guarantees but odds are you're looking at pressure generating from the hydraulic side, not the air side. While it's locked, check for air pressure at the driver's side glad hand WITHOUT stepping on the brake pedal. Again no guarantees, but no air release there is another "kind of sure" diagnosis that it's on the hydraulic side. If both of those point the same way, then you have a prudent direction to take your diagnosis to the hydraulic side first. Make sure there is at least half an inch or more of free play in the pedal linkage to the master cylinder, AND the pedal return spring is in place and lifts the pedal firmly until it hits the underside of the cab floor. The "valving" if you will inside the master cylinder will act like a one way valve if the master's not returning all the way. Fluid will be able to "drop" to make up for what you've bled, but it won't open the "fluid vent" so as the fluid warms and expands it can't go back, the brakes drag, making more heat... Ditching a little fluid will solve the problem temporarily.
If there is air on the service glad hand (driver's side) with no brake application, that is an indication of an air side problem. In the air pack, the vent is restricted, something along those lines. Finding that (and it'll probably be a little "burst" versus a full stream, and probably at way less than system pressure), that would send you to chase the air side of the system.
