Brakes overheating after driving a while

When this happens are all 4 wheels locked up?

Next time this happens pull the vacuume nipple (not just the hose, the nipple has a one way valve in it) out of the Power brake booster, if you instantly have normal brakes the booster is the issue.

I would not keep trying to drive if you start deeling a drag, you can destroy rotors and pads and calipers.