04 Express 2500 Rear Disc Brakes not working
 

Not my daily driver. Rear calipers fully free and sliding on pins nicely.
Parking brake works. 104000 miles. Rear wheel drive.

I rotated my tires yesterday, and noticed that both rear brakes do not seem to be energizing...just a small wear trace on rotors. Removed calipers from carriers...pistons not seized. Pushed brake pedal, small amount of piston movement noted, but didn't seem like enough. Re-installed calipers and cracked loose bleeder screws...no air, just fluid. The rear brakes don't seem to be applying/energizing. ABS lite present during bulbcheck only. No MIL's. Any experienced input would be appreciated.

Put the van up on a lift. Have someone watch, put in drive, accelerate, brake and see if the rears stop... that's the only way you are going to know for sure.

Did a gravel road test. Skids in the front, NOTHING in the rear. ABS didn't seem to be doing it's spastic thing on the fronts, either.

Master cylinder issue? there is a float in the system that separates the two systems....

Master cyl is what I was thinking, but thought maybe some ABS issue, or load sensor might be causing it. What is the "float" you speak of?

There is a valve that with equal pressure on both sides stays in the middle. This allows fluid pressure to both sides of the braking system. Front to back I think. If you loose pressure and the valve slides over to block off the plow pressure side, then it might be stuck there. That would sewer your brakes on that part of the system. Check for a leak..

Thanks - I forgot about the float valve. How can it be "re-centered" if it has slid out of position? I don't believe they have a centering spring, do they?

It is supposed to self centre once equal pressure is supplied once the leak is fixed. However I am not posetive on that.
You could give a brake shop or the dealership service folks and ask.
Once that piston slides over it is supposed to activate the brake warning light. Is that light on?

No warning lights are on