The rudder pedals are designed so that the ball of the foot rests on a flat part of metal, which is above where the pedals pivot to apply the brakes. Thus any pressure on the rudder pedal will necessarily put pressure on the brakes, even if the pilot is careful to rest his foot on the bottom part of the flat area.

RV-8 builder Jeff Jasinsky designed extensions to allow the heel of the foot to apply rudder pressure, which is below where the brake portion pivots. This allows rudder to be applied without applying any brake pressure. Randy Lervold has pictures and a description on his web site.

I plan to mod my brake pedals like this.