Is this a Brake Booster Problem?

I recently found out I had a leak in a brake line. I took it to a mechanic and they replaced the brake line for me. ($700). Then I got the car back and now my brakes are making a hissing noise when I apply them. The pressure is very good in the pedal. I don't have to press down much for the car to stop. The mechanic told me "the brake booster is probably going to go down the line". What the hell does that mean? Is is dangerous to be driving it like this? It's a 2000 pontiac sunfire. The car stops fine but its hissing from the pedal area. I can't afford to get it fixed right now and am wondering if it is dangerous to drive like this or if I can go a couple of weeks. If you have an explanation for why it could be doing this, that would be great. Also, a ballpark for how much it would cost to replace.