I have seen frozen brakes in my '95 LC and '94 4Runner and determined that it was the booster check valve or some booster part that had frozen. It only happened when very cold and I don't think it was fluid related. YMMV - I might still use this as an excuse to bleed the brakes!
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner