My bet is that it is not a pulse in brake pressure but a vibration felt in the pedal. The wiper motor and pedal assembly are both mounted to the cowl/firewall. As the wipers move back and forth you feel the pulse/vibration from the wiper motor in the pedal.
as a matter of fact i was just about to post the same thing cause it is raining today and i paid attention- all i felt was some vibration from the wipers. hell, the whole truck rocks side to side with the motion of the wipers so i can deal with a little pedal vibration. pulsating pedal is something entirely different.