Greetings :-) I recently found that to apply my brakes I had to push them all the way to the floor.

Greetings :-) I recently found that to apply my brakes I had to push them all the way to the floor.I checked the brake fluid and found that it was low. I added some and after several hours I checked it again (due to brakes still not working properly) and it was again without brake fluid. I suspect a leak, but there was nothing under the car on the ground and nothing dripping. What could be my problem and where do I go to get it fixed? I have a 98 chevy malibu ls. About how much will it cost? Thank you in advance for any assistance.