Check for any brake fluid leakage under the car. Then check the fluid level in the master cylinder. If it's low, fill it up and pump the brakes until you can find the leak. If the master cylinder is full, and you're satisfied there's no leakage, the problem is likely a bad master cylinder that blew an internal seal. Post back with what you find.

__________________
95 E320 Cabriolet, 169K