Numerous times this winter, I have found myself standing on my brake in order to stop the car. This has only occured on first starts (after the car has been cool for 8 hours) and takes only 30 sec or so to return to normal. Thinking that there was water freezing in the line I took it in for a brake fluid change. Doesn’t appear to have corrected the situation. Could it be something to do with the power assist? I have read that vacuum lines or check valve could be the culprit, but I would think that the problem would be continuous as opposed to only extreme cold weather.
Your booster needs to see a good vacuum before it does it’s thing.
Anything that delays or prevents your manifold pressure from dropping to a good vacuum (such as cold, slow acting check valves) will affect the booster.
You need to determine if the problem is related to the booster or the brakes.
Moisture in the workings of the calipers may require increased braking effort if it’s frozen and restricting their movement.
Does this clear up after 30 seconds of pumping the peddle without moving? If so, it points to the booster.
Does it require 30 seconds of braking while in motion? If so, the heat generated by the brakes may be thawing ice trapped inside the sliding mechanism of the calipers.