Omg...help

[PKRWUD]

Your update supplies the answers, and confirms my first reply.

You haven't bled the brakes since replacing the master cylinder. Bleed them.

The pedal becomes rock hard when you hear the hissing. There is a leak in the booster diaphram. Ever notice how hard the pedal becomes when the engine is shut off in a car with power brakes? That's because vacuum is no longer being applied to the booster. The hissing you are hearing is the vacuum escaping from the booster, and the hard pedal is the result. The Power Brake Booster needs to be replaced.

First replace the booster, then bleed the brakes. All will be well.

Trust me.

Take care,
-Chris