So I just swapped to rear disk brakes on my third gen. I installed a 4th gen master cylinder and a new adjustable proportioning valve. I made sure there were no leaks and bled the brakes all the way. When the car is on and there is vacuum running the pedal is very soft and makes a hissing sound but the car seems to stop ok. Is this a result of the booster going bad or do I need a 4th gen booster since it's a 4th gen master cylinder.

 

Sounds like a bad booster.

 

Rumor has it, you need a 4th booster w a 4th gen master. And I agree, hissing sounds like a booster

 

I've used 3rd gen boosters with 4th gen masters on a few cars, works fine, no issues. The problems can arise when using 4th gen pedals in a 3rd gen when using a t5 or other swap transmission that isn't a t56, the pin on the clutch pedal is in a different place and creates problems as there is not enough throw