When pressing the brake pedal there is a noticeable whoosh and the brake pedal goes all the way to the floor?

When I press on the brake pedal, it makes a very noticeable whoosh and goes all the way to the floor. If I pump the brakes, it doesn't get any better. Keeps whooshing and going the floor. My mechanic has bled the brakes two or three times over the past year. It improves slightly for a short time. He says the brakes are good and that he was told in a course for safety inspection certification that early 2000 Chevys had a lower brake pedal clearance. I can't believe that "lower" means all the way to the floor. It makes me uncomfortable driving the vehicle. Any suggestions for what might really be the issue? I'm a senior citizen female without a lot of car knowledge but this just doesn't seem right.