1991 integra brake fluid problem

need some help.
my girlfriend's uncle discovered her 91 integra's brake fluid was OUT. there was none and he didn't know how she had been driving it w/o crashing. anyway, what could be the problem? and how much would it cost to be fixed? need to know ASAP. thanx.

Could and most likely is a leak. I would start by looking at the clutch master cylinder. Look under the hood at the thing that is under the resovior. Also look under the dash by the pedals to see if any has leaked inside there as well. If no leaks there then start looking at all the hoses where the hose runs from the caliper to the body of the car. Start with that and tell us what you find.