Crap. What to do to fix if it is ABS Accumulator?

If I understand you correctly, you're saying the pedal is hard and doesn't travel as far as it normally would to engage the brakes, and only when cold outside? Something exactly like that has happened to me a few times here in Minnesota winters. It happened on some of the coldest of cold days, like -30F or lower. The pedal would barely travel at all and get real stiff, and no brakes. My guess is the master cylinder, brake booster, and/or pedal assembly was simply freezing up in the extreme cold. Once I "broke" it loose or the truck warmed up in the engine compartment or inside the cab, there was zero problems.

If only the front brakes were locking up, or if only the rear brakes were locking up, you'd still have braking ability. But you wouldn't lose it completely. If you had air in the brake system, the pedal would be very spongy and travel all the way to the floor.

Though I don't have ABS, the master cylinder, brake booster, and pedal assembly are the only things that can affect the entire braking ability.