I've driven a single seater at PalmerSport, it was a 3ltr V6 Formula Jaguar and the force you have to apply to the pedal to brake is phenomenal.
It had a little light on the dash that came on when you applied enough pressure and believe me, it's a lot.
Even so, it'll be nothing to what they have to apply in an F1 car.
http://www.palmersport.com/formulajaguar.aspx