i have run into frozen wheel cylinders so many times i lost count. and to make matters even more interesting, this also happens on daily drivers as well. when the fluid gets moisture in it [the fliud is "hydroscopic", which means it absorbs water] and turns that nasty brown/black color, problems can, and do, occur. this applies to DOT 3, 4, and 5.1 fluids only. DOT 5 doesn't absorb water because of it's synthetic nature, but i have also experienced frozen cylinders [although rare] using that because the cylinders had collected water at the bottom of the cylinders, leading to rust in that area only, just enough to make it stick. after taking the cylinder apart, the area could "usually" be polished out without making the bore out of round, then using a kit, put back in service.