Q: Brake fade after hard driving

Basically I've noticed, after running the car hard for a bit, that I get a bit more brake fade than normal when they're running really hot. I first suspected it might be the fluid, since I haven't changed it in over a year, but I'm not so sure at this point. I've noticed that when pumping the brakes while the car is on, then resting my foot firmly on the brake pedal after it stiffens, the pedal will slowly drop to the floor. Not really all the way to the floor, but it does drop quite a bit. My experience, this is usually bad fluid, or possible master cylinder letting fluid slip by.

Now, I don't have any fluid leaking around or under the master cylinder, nor is there any fluid on the rotors or lines. Pads and rotors were done about a year ago. Fluid seems to be at the top of the res. If the car is off, I can pump the brake pedal without it dropping to the floor, so maybe it is the fluid, but... I dunno, seems kinda unlikely I boiled it, and I don't see how air could've gotten in there.

My car has 40000 miles.
My car has an automatic transmission.