***Your car is not safe to drive***
The rear brake lines may have rusted out (just happened on my sister's Mystique), drum brake wheel cylinder(s) have failed or disc brake piston is leaking. If it is all 4 wheels leaking, maybe the bleeders are loose. I would expect to pay a couple hundred US dollars (mostly labor) to fix your car, less if it is just bleeders. You could probably look under the car with a bright flashlight around the back sides of the brakes and anywhere there is a spot on the ground, and find the leak.
