I realize this is an old thread, however I found it very useful for diagnosing and repairing my Korky 528 valve in my 4 year old Toto. I too had an intermittently hissing valve and no apparent leakage which stopped if I shut off the supply valve. I replaced the valve cap after confirming the old cap gasket had failed. It appeared to do the trick...then 2 days later the valve began hissing again. I took it apart, reassembled and it still hissed. I thought I was going to have to replace the whole valve when it occurred to me that it might have something to do with the float. I noticed when I connected the float on the new cap it was very loose compared to the way it fit on the original cap. It's held onto the cap by spring tension of the two plastic hinges on the either side of the float brace. I found that I could affect the valve action dramatically by jiggling the float while it was assembled. I took the float off and (gently) spread the plastic hinges away from the sides so it fit tightly when placed on the valve cap. The valve is now working perfectly. I hope this little tidbit of information helps if someone is still experiencing a leaking valve after replacing the valve cap.