Excessive heat is probably the cause of the cracking. I had the same problem with my front brakes. They would sqeal all the time and were glazed and cracking. Even right after new pads. It was the master cylinder residual pressure valve. When you press on the brakes it puts pressure on the lines and then the caliper (or in your case the brake wheel cylinder) causing it to slow the truck. If this valve is broken or clogged, it does not release all the pressure in the line when you get off the brakes and causes it to drag and get really hot. I ended up replacing every thing. Master cylinder, calipers, rear wheel cylinders and flushed all the lines just to make sure there was no crap in the lines. If there is crap in the lines it can cause that valve to stick, or even cause calipers and wheel cyliders to stick. It can also plug up the proportioning block, and cause NO pressure to get to the brakes! I have had 3 vehicles with sticky brakes problem. One was a caliper sticking, one was a master cylinder, and other was truck problem described above. In your case, since it is both rear brakes and you dont use them under heavy loads, I would guess a proportioning valve, or the master cylinder, or parking brake is dragging. Good luck!

 

Well, as I understand the phenomenon of linings cracks.
1. Has no effect on efficency of brakes, unless from O'heat, which is apparent.
2. Many times is apparent on new linings. Bonded linings are pressed to metal under pressure and heat. The cracks occur because the material is bent.
3. Process continues when in use and cracks later appear if not present when installed.
JMHO

 

When I put the drums back on I left the axle shafts off and backed off the the
hand brake where the two cables connect to the one cable going to the cab.
Next I adjusted the star washers so I could just feel and hear the shoes starting to touch the drums. Pushed the brake peddle a couple times to center
shoes to the drums and rechecked the feel turning the drums.
Next adjusted the one brake cable connecting to the two drum cables and rechecked the system. The last thing I did was put the axle shafts in the drums and install the tires.

Someday I am going to get new riveted lining instead of bonded and maybe not have to hear squeaking every time I apply the brakes.

P.S. Thanks for your thoughts.