Your car's master cylinder should have break fluid in it, but the brake booster shouldn't. If the brake booster has fluid in it, you have a master cylinder problem.
Overall, the master cylinder is low maintenance. However, when it begins to wear, you may notice that if you keep your foot on the brake pedal, the pedal goes almost to the floor. In addition, you must pump it to regain pressure.
Inside a master cylinder are O-rings, which keep fluid from leaking out. However, when they wear, fluid may leak past them. Because the master cylinder and brake booster lie in tandem, the leaked fluid may drip from the master cylinder into the brake booster.
Monitor the fluid level in the master cylinder. If you must add more than an ounce or two, have your mechanic check the master cylinder. You do not want to postpone any possible repairs your break system may need.
