overfilled master...dragging brakes

[ajcbgray]

So, I had a caliper in the back start to drag yesterday, so I replaced it today along with both rear hoses and the rear pads. Bled out the brakes and everything seemed fine. Piled the family in the car and drove about 3 miles and the car started feeling very sluggish on the interstate. As I started looking for an exit I could smell the brakes and as I was exiting I could see smoke. I went about two more blocks so I could pull onto a side street and out of traffic and was having to mash the accelerator just to go 10 mph or so. The rear brakes smoked for quite a while as we waited for a friend to come pick us up. I went back an hour or so later and the brakes had freed up, but I checked a few things first. I had overfilled the master cylinder, which I have read can cause the brakes to drag. I drained some fluid out and drove home without any problems (though I didn't use the brakes until I got within a few blocks of my house.)

Can anyone explain to me why an overfilled master cylinder can cause the brakes to drag?

thanks,
ben