Get the back end of the vehicle in the air.
Have an assistant apply the brakes as you observe how the rear brake calipers react by turning the rear wheels by hand, pedal on, pedal off. I'll bet the wheel locks immediately when the brake is applied, AND the wheel still doesn't turn when the brake pedal is released. After say two seconds, the wheel slowly begins to turn.
I say the calipers are not releasing in a timely manor.
This would indicate a fluid restriction, given the pressure of the brake pedal can easily overcome a restriction, whereas a restriction is NOT easily overcome when the brake pedal is released.
Inspect the entire metal brake line, front to rear for kinks and pinched line, especially the most rearward lines. Did someone drive the vehicle over a curb or drive off road in the last 24K miles?
If you see nothing amiss on the lines, we now focus on the calipers. Remove the wheels, repeat the above test.
Observe the movement of the pads. Look for torn or missing rubber boots on the caliper pistons, and rust buildup on the pad assemblies where the pins attach to the pads.
Remove the pads and carefully apply the brakes just enough to expose a portion of the pistons to look for rust on the piston sides.
Lastly, pads in place, wheels off, apply the brakes. Instead of releasing the brake pedal, crack a brake hose at the caliper(or open a bleeder valve), and see if the immediate release of brake fluid at that point immediately releases the pistons.
I'm confident you have a restriction in a brake line, not bad calipers, and I'm betting it's the hoses, despite the fact you changed them
K. Interior rust on those hoses WILL clog and restrict brake fluid.
As Jayare pointed out, the metal lines won't rust shut, they would rust thru causing a leak, but I assure you the hoses DO rust and cause these issues. Can they rust shut in only a short time frame? Sure, if they are not MB original or high quality(stainless) products.
One of the problems with brake fluid is that it is hydroscopic, meaning it attracts moisture, and that can and will rust the interior of brake lines.
Another thing to consider. If there is air in the caliper, it can heat up under heavy braking(or pads dragging on the rotors), causing the caliper to apply the brakes slightly, even when the pedal isn't applied. I doubt this is the case, it takes a lot of heat to produce these symptoms, and you'd probably smell the burning, but it's a possibility.
A malfunctioning check valve, or proportioning valve would give these symptoms as well. I personally have not seen this occur, but I would consider that a possibility.