TOTO Thermostatic Mixing Valve Issue

[Philip in Connecticut]

I have a TOTO TSTA single volume control thermostatic mixing valve in the master bath. I installed this maybe 10 years ago. The problem: recently, it started to leak from the shower head. It is a very slow leak: water builds up in the shower head, then it comes out in a flood (assuming surface tension of water in shower head has something to do with this). This cycle repeats, although it can take hours for the volume of leakage to build to the point that it finally drains out. I have replaced the valve cartridge at least 3 times, but no luck. The first two cartridges were installed without plumbers grease on the o-rings, per toto tech support. Today, I removed the cartridge, cleaned the inside of the valve body where it contacts the external o-rings on the cartridge with a valve brush (stiff nylon bristle brush) and then installed a new cartridge with plumbers grease (silicone). Still leaks. The one thing that concerns me is that there is a green buildup (I assume brass oxidation) on the mating surfaces of the valve body where the o-rings mate to the valve. My attempts at removing this buildup with the brush did little to remove it. Could this be the issue, and if so, how to safely remove the buildup with damaging the brass valve body? Thanks for your help!

 

[Philip in Connecticut]

Anybody?

 

[Sponsor]

 

[Terry]

Perhaps something to loosen the buildup, like WD40 spray oil.

 

[Philip in Connecticut]

OK, good idea. I took one of the older cartridges apart and I see there are two o-rings inside the cartridge, which I assume control the on/off function (since its a dual use valve, for my specific variant, I think only one of the flow channels is used). I do wonder where the leak is: inside the cartridge at one of the inner o-rings, or at one of the 3 o-rings outside the cartridge. It is a puzzle. Thanks for your help.