Master cylinder or booster?

My car recently showed a warning light for the brakes. I checked the fluid level and sure enough, I was just about empty. I bought a bottle of fluid and mentally prepared myself to patch a leaking line within the next few days. It's now a little more than a week later and my brake fluid level has not changed at all. I have been keeping an eye on it regularly while waiting for my driveway to dry up. Here's where it gets goofy, I did some pretty hard braking the other day and aferwards my pedal started giving way as if the master cylinder was bad or I had a leak that wasn't very large. Since I had already had to refill my master cylinder I assumed that I had a leak that got worse. This is not the case my fluid level is still right up top, but it does feel like my master cylinder is bad. the reason for my question is that I noticed, while pumping my brakes like a madman to try and generate a puddle under my car so I could determine where the leak could be, a bit of a hissing sound that went away once the brake pedal reached the bottom of its travel. A brake booster with a blown rear seal will hiss when the brakes are applied but in all my experience does not stop when the pedel stops moving. Does anybody know if this is normal? I know I have to replace the master cylinder now, I'm just wondering if I need to replace the booster as well.