What you just posted is textbook description of an internally bypassing master cylinder. Id start by replacing it

 

A little bit of that when you hit it is normal (every car I've had makes a slight initial hiss if I hit it) but it shouldn't be constant.

 

its just when i initially press on it.

 

After putting my car back together a connection at the ABS block was leaking just enough to make a hissing sound when I pressed and released the brake pedal. Locked them all down and no more hissing sound or squishy pedal.

Might want to triple check those connections. Free and only takes a few minutes with the right wrenches.

 

I think it point to a problem with the master cylinder probably it need to be replaced.

 

Sounds like you got a bad master, especially with it slowly sinking to the floor under consistent pressure.

 

How many miles on the car ? Sounds like a master to me .