Toilets, Sinks, Showers, Tubs and Disposals - WC mixing valve leak

I installed a hot water mixing valve specifically designed for the water closet tank to prevent sweating during warm months when humidity is high. I purchased it from HD and they don't carry it anymore. The problem is (for the first time in 2 years ) I adjusted the screw to shut off the hot water because it isn't needed during winter months. It has developed a leak through the adjusting slotted screwcap. I drained the system, and took it apart. It has (2) O rings. One for the nut that allows the piece to be removed and the other for the shaft of the screw that goes through the center of the nutcap. I checked each closely and don't see any apparant nicks in the O ring. I still cleaned the brass part with alcohol and Q-tips, then lubed it up with plumbers grease but it still leaks. I backed it out to slow down the drip and would like to know if I'm missing something ? Also, do the make a rebuild kit for this. One O ring is really tiny, and I doubt even a plumbing supply house will have this.

When you screw it all the way in, you shut off all the hot water?...except for the dripping out around the shaft? Is that correct?

If so, that almost would sound like a 2 part problem. One being that it doesn't seat when closed. The other being that the o-ring(s) don't seal when open/partially open.

O-rings can wear you know. They don't have to have a nick. How small is small, regarding the small o-ring? Can you measure the i.d. and o.d. of it?

The id of the small one is about the size of a ball point pen refill. I think the reason it leaks a bit faster when screwed in is that the cold water in the mixing area would be around the shaft more or the threads start to go all the way in. This has no washer type seat, its only a machined brass seat...Brass on Brass. I'm tempted to solder it open and put a ball valve on the hot side for seasonal shut-off.