[msc]

Your brake booster may be fine. The seal between the master and booster is bad causing the hissing at the master. This bad seal will also allow the pedal to rise when the vacuum source is removed (engine stopped). The main reason this seal would be bad is if someone replaced the master or booster without putting a new seal in.

The pedal sinking when holding the brakes on is either a bad master cylinder or a leak in the system (possibly at the master).

Are you loosing brake fluid? If yes then it's a leak and could be at any of the calipers or the end seal on the master. If you can't find signs of leaking at any of the calipers a likely leek location is the end seal of the master letting fluid escape into the booster. If you are not loosing fluid then it's a bad master leaking from one internal chamber to another.

The surface rust stream is probably caused by this or a previous master leaking fluid out the end seal. One thing to watch for when replacing such a bad master is to see if there is any brake fluid inside the booster that would brake down the diaphragm in the booster over time.

Mike