When I drive first in the day, halfway down my hill, the break pedal becomes stiff and it feels like the truck is hAving to work hard to move forward. Like its pushing something. But when I pull over, put it in park and restart the car, it's better. It happens at least 4 times each day on the way to work. I had the mechanic check the calipers and there is no ceasing. What's is it? Abs? Tranny? Help