85' bronco brake question?

Looked at a bronco yesterday that someone is selling on craigslist and i took it for a drive and when i pressed the brake down it makes a big air hissing noise. i asked the guy why and he said it had air brakes. hes he telling the truth or does a hose have a hole in it or the booster going out? if so how much would a new booster be. im really interested it the bronco