Today I noticed that if I depress my pedal firmly, there is a hissing sound "ppppssssssshhhhh" coming from behind the pedal at the booster. Soon as I ease up on the pedal the sound goes away. Question is should I take action on this immediately, or should I just hold off until it gets worse? The car feels like a choo-choo train while braking at certain speeds (no steering wheel vibration at all), otherwise it does alright.

 

Mine only hissed when I let go of the pedal. I didn't notice any loss of braking power, but in your case, it looks like it's losing pressure when there's load on it. Under emergency braking, it could leak the pressure and not provide 100% braking power. I say get it looked at unless you feel comfortable. Mine was hissing for like a month until I got it replaced.