I did some playing around with a bathroom scale and the brake pedal in my truck one day. If I recall, 40-50 pound required very little effort, and 70-80 pounds of pressure was fairly easy. Effort was required around 120 pounds, and 150 pound was starting to lift me out of the seat. Kind of told me I'd prefer threshold braking to be below ~120 for road course work. A lower seat to brake pedal angle would have helped.

That being said, F1 drivers use approximately 190-200 pounds of force at threshold braking.