Like I said in the last couple articles, if you don't flush and bleed your brake fluid system regularly, the brake fluid itself is designed to absorb moisture and in time dirt and grime start to build up within the system. In turn, the seals start to dry up and deteriorate which causes the brake fluid and pressure to leak.
Whenever you pump your brake pedal, your brake master cylinder's job is to push the brake fluid throughout the system to push the brake calipers pistons and cause the brake pads to squeeze the brake rotors and cause the car to slow down or stop. All the while the brake booster gets air from your engine's intake and use it as a pressure to help the brake pedal push the brake master cylinder with ease.
The common symptom of brake master cylinder failure is a mushy feel to the brake pedal, deeper brakes and loosing brake fluid from the reservoir. The brake booster on the other hand, will give you a very stiff feel and very hard to control braking. Although