What would happen if you drove your car after adding power steering fluid to brake fluid?

If the fluid causes the rubber in the Master Cylinder, you can lose your braking ability. The rubber seals in there are designed to cause pressure one way and allow the oil to pass by when you release the brake. If that oil cant get by because the seal is swelled, the fluid will not compensate for the wearing of your brake pads and your pedal will slowly get closer to the floor as your pads and/or shoes wear.