Brake Problem

I have a 1991 Chevrolet Corsica. Our car sits for a while especially in the summertime while my wife is off from work. I recently went out to start it to see if it would run okay. It started and ran as usual, but a funny thing happened. When I put my foot on the brake pedal and got ready to shift it into gear, a sound like air slowly letting out of a balloon came from the driver's side probably around the firewall area. Also, the brake pedal went to the floor. As I hit the brake pedal, I could hear air coming from the rear brake area on the driver's side. I turned the car off and started pumping the brake pedal to see if it would stiffen up like it's supposed to when the car is off. It didn't, instead I was still able to pump the brake pedal and it was going toward the floor like it was when the car was on. I suspected the power brake booster at first but when I checked the master cylinder a day later, it had lost some brake fluid. Quite a bit, I might add, but I can still see the fluid. It was just too low than it should be. Any ideas what this might be? I've got to get this car in for a stupid e-check about mid September so I would greatly appreciate some feedback please. Thanks in advance. :iceslolan

Sounds like a rear line blowout out to me. Check all wheels but especially the rear ones and look for the rear rubber hose. The rubber hoses can fail all at once and rupture. Would have to repair and bleed the brakes before you would have pedal. Otherwise I would think it would be a wheel cylinder or a rusted steel line. Pull all wheels if you do not see leaking rubber hoses.