Great question. I've never experienced it before so it's a little hard to describe. Pedal feel goes from hard (lots of resistance pushing back on your foot while pushing down the pedal) to soft (almost no resistance pushing against your foot). The brakes seem to work in slowing the car down until the brakes are 100% depressed, and then it feels like they aren't holding. Admittedly, I've only intentionally pressed the pedal down that far twice since I don't want to break something more than it is already broken.
As silly as it sounds, it's almost like I've pushed the pedal further than it should travel and that causes some pressure valve or something to release.
The pedal does come back up on it's own. If you depress the brake pedal fully, and then let up 50% and then try to depress it back to 100%, there is very little resistance the second time compared to the first.
The hiss only happens when pushing down on the pedal. It sounds like there's air in the lines getting compressed - it sounds very similar to a "whoosh" sound as the pedal is pressed.
Given what you've written it probably not a brake booster. Could it be a master cylinder? Maybe I fucked up installing the new rotors/calipers/lines? I recently had my clutch slave cylinder replaced and they bled the brakes at the same time but this issue popped up after that.
Thanks for the help