Q: The brake pedal is low and hard to press, and my car does not brake well

Hi:

I have a Lanos Daewoo 2001, in mint condition only 27,000 miles. A few days ago, i was getting to drive the vehicle and when i pressed the brake pedal i heard a pop sound, the pedal was very heard. i pumped the pedal a few but it stayed high and hard. i drove the car home using the emergency brake and the brake pedal. I was able( decelerate) to stop the after pumping the pedal a few times. Every time i pressed the pedal i heard like an air leak sound. Next day i noticed the brake pedal was working almost like normal, however it was a bit low. I tested the brakes and I was able to stop the car but as i said the pedal was low. i checked the brake fluid reservoir and it was at the normal level, also I checked other areas and did not find any leaks. This issue is a quandary, I have no clue. Is the booster failing or a bad caliper?? Your response will be greatly appreciated. Thanks

My car has 27000 miles.
My car has an automatic transmission.