Rear wheel cylinder blew out on the way to work today

I got the wagon to stop using the E- Brake. But I need to fix this. I have had a slow leak from the master cylinder for a while so when the pedal felt soft this morning I figured it was just low. But then the pedal hit the floor no matter how much I pumped it. I hit a guy in the bumper at less than 5 mph but he was cool about it. I tried adding fluid and saw a big puddle at the rear drivers wheel. How hard is this to fix on my own. Kragen has the cylinder in stock for $9.99.

Thanks
Tim