So I drove my car for the first time last week after sitting since September. The brakes are pretty bad. It doesn't stop like it did last year.

The brake light is on aswell. The reservoir is pretty loose on the top of the master cylinder and when you move it you can see a little fluid.

So I planned on replacing the master cylinder and flushing out the whole system.
This would help, correct? And how much fluid does the brake system need after you do a flush?