Could be any number of simple things, or a combination.
Hopefully, you are test breaking on a straight flat portion of concrete and not in the same location every time.
Problem could be brake line expansion, sticky caliper, low tire pressure, bent suspension link, bad suspension bushing, clogged brake line, warped rotors, bad master cylinder, bad rod in master cylinder, loose pedal connection to the rod, stupid stuff like that.
I had a guy complained of such braking under all conditions on his Bronco, come to find out, his X pinched his front brake line with a snap pliers. Since he didn't do anything for some time, the brake line hardened like that and he had to replace it. Go figure.
BTW, same X also busted out his front headlights and he wondered why they didn't work when he turned on the switch. Doh!!! He was young and in-experience. LOL.