Both rear brakes on my '04 350 start dragging when they get warm. Pedal also gets stiff. However, when they cool down, Everything is normal until they heat up again. If it was just one side, regardless of temp, I'd think either collapsed line, sticking caliper, E-brake not releasing, Etc... But both and only when hot? Has me a bit stumped. Have not pulled them apart yet. Actually in a parking lot waiting for the wrecker to show up... Only has 52K miles at that and down south.

 

The rear brakes do have a common flexible hose, more or less above the diff pumpkin.

 

Looks like 2 flex lines go into a valve on the axle then off into steel lines to both calipers.

 

I'm having the same problem. After a few miles of driving the rear brakes begin to drag. When I've stopped to investigate, I smelled burning pads, see both rear calipers smoking and hear them releasing. Didn't feel any difference in the pedal. Only thing I noticed was a wet spot on top of the differential where the rear brake lines meet. Also my brake fluid is lower than the low level limit.

 

My level is right at Max and pedal gets firm but other than that, you described what I have perfectly.

 

Did you figure out the problem?

 

Pulled left rear wheel off last night and "flushed" that line with new dot4. Dumped what I pulled in to a coffee pot. Black as tires with a tint of green in bright light and was sparkling. Kept reservoir full and did the right rear. After two runs, came out clear yellow like new. Put wheels back on and started her. Pedal is much firmer already and takes almost no effort to stop from rolling out of the driveway. Stood on the pedal and held for a minute. Tried to free roll again. Nope. Rocked it a little and whatever freed up. Repeated and nothing happened. If I'm not mistaken, this truck has 4WABS. And both rears locking simultaneously tells me it's a ABS issue but no pedal shudder or sounds from the pump. Tonight I'm probably getting under and inspecting the lines. I already seen I need new pads as one side they have cracks and gray possibly white spots as do the calipers. I'm at a loss here and need someone who is familiar with these trucks brake system.

 

Look up how to test the ABS pump on here. This is what my issue was and had to replace.