Symptoms: Periodically, the brakes will become very soft and the pedal will nearly go to the floor before I can feel braking power. When this happens, I let the pedal come back up and then give it a quick thrust and then it'll stiffen up. I've found that it more than often goes soft when I brake very casually / slowly. However, if I always begin braking by giving it a quick thrust, it seems to be fine.

Possible Solution: After taking it to a couple places, the mechanics said they believed the valve in the master cylinder is cracked and thus not holding when I brake slowly. So, I purchased a master cylinder.

How would I go about changing it out? I've been told it's not a complicated procedure and if done well I might not even have to bleed the brakes afterward. Will I require any special tools, etc.?