I noticed that when my brakes are hot, they are more likely to lock up when they are hot, lets say below 20 when Im braking hard.

When the brakes are cold, I can slam on the brakes at 30 MPH and have it come to a complete stop without lock up, a little warmer they stop better, then when they really heat up they lock up under hard braking.

How does heat affect grab from the pads to the rotors, and the tires? It seems when the brakes are hot they stop better, as long as Im not locking them up.

I had new calipers, two new rubber lines, new guide pins, 1 new steel line, and a $60.00 pair of Raybesto's Superstop pads in the front, and then Bendix premium linings in the rear which are adjusted and working great.