Brake Drag

[ybk at dongames.net]

After driving for about 20 minutes on a freeway brakes start to drag. The brake pedal play diminishes to virtually zero. It is not a hard lock up but pretty noticeable. Fortunately both times it happens very close to the end of the tip. After several hours it is all back to normal. All of this started after replacing rear calipers and changing brake fluid.

Is this a master cylinder acting up or something else?

Eugene.