I have an problem with the brakes after the ABS has been activated on snowy roads the brake pedal drops about 75% and stays there for quite a while. Eventually it comes back up to a regular pedal again, until the ABS is activated and then the pedal drops again. The last time I lost the pedal it hasn't come back up to normal for 2 weeks now. The brakes still work fine, just a very low pedal. I have bled the brakes once and the car had new pads and rotors 6 months ago
A friend of mine just had his ABS unit replaced for this problem. We therorise that the dump valve is stuck open. Because the valve is open the break pressure is exposed to the accumulator. During an ABS stop the accumulator bleeds access pressure into the accumulator. The accumulator is in essence a piston and spring. When the ABS event is over the dump valve will return fluid back into the system. If the dump valve is stuck open then the breaks won't engage until the accumulator piston bottoms out.
You can try having a shop preform ABS bleeds by activating it with a scan tool. This may help but probably won't.