I searched the Korky site, and the internet in general and could not find the answer to the hissing and leaking Korky valve, so I thought I'd pass on my solution.

Certainly if the red diaphragm is bad it will leak but after replacing that the leak is still there? On my units the leak was coming around the CENTER TUBE. There is an o-ring on the main body of the unit, through which the CENTER TUBE inserts. In my case the o-ring lost its elasticity and caused a leak.

Since these leaking or noisy Korky valves are so common I'm surprised I did not encounter any mention of that o-ring. That o-ring certainly is important because it has to hold back the supply line pressure.