The drivers really have to stamp on the brakes with every application, almost standing up in the car to do so. On road cars, servo-assisted brake systems multiply the pressure you apply to the master cylinder but the regulations in Formula One demand that the braking force has to be generated by the driver alone.
They need very strong legs to do this, but they do get some help from the violence of the braking manoeuvre itself. The cars decelerate at around 5G (compared to the 1G we might see during an emergency brake in our road cars).
At this deceleration, their leg will weigh approximately 100kg, and the weight of their leg on the brake pedal provides its own form of servo-assistance to help them - the harder they press, the more the car slows, the more it slows, the more their leg weighs which helps them to press harder.
What is remarkable is that in the midst of all this, while pressing the pedal with well over 100kg of force, the driver is required to modulate his effort on the pedal with all the delicacy of a concert pianist in order to coax the car through the corner at the very limit of what the tyres will permit - it is a delightful contrast of violence and gentleness.
This answer is a snippet from an article on F1 Brakes on the AMG Mercedes website. See the full article - here