I have seen videos of a mechanic running in some solvent/cleaner through the ABS valve body. Not sure what clear fluid it was. The mechanic doesn't want to reveal what it is but says 7 out of 10 Tenere's he's attempted to fix is successful. From the internet, some use denatured alcohol aka ethanol and compressed air to frees up sticky valve. These were ABS for a BMW M5.
Researching further on why the valves gets stuck or sticky, I came upon pictures of copper plated valves; where this would be abnormal. The valves are not copper plated but probably just stainless steel. With the plating, the diameters gets larger and is probably the cause of sticky valves. I also read that the copper comes from corroded brake lines that released Copper ions that are normally neutralized by good brake fluids. Once it reaches 200ppm of copper, there is a need to change the fluids, else the corrosion will accelerate. Now these free ions, when are not neutralized, goes onto any metal parts within the lines and starts plating them, with the presence of moisture (electrolysis).
I read about how to remove Copper plating and 1 part Hydrogen Peroxide mixed with 2 part of white vinegar could strip the plating. So if this is the method to use, it's best to just soak the Valve block passages only. Not been done on ABS valve block that I know of, so there is some element of risk what this solution could do.
My situation is a hard rear brake pedal, sometimes it recovers with full ABS function, normally after a 20mins ride (but requires to off and on back the bike and then do not use the rear until the front brakes are pulled quite hard a few times). Probably when the temperatures heats up, the UBS valve frees up. So far, I have done about 10X cycling with bleeding. Every 3 pedal pushes to bleed, 1X cycling. No joy, still sticks on the rear. A hard push on the pedal helps to gain back rear braking but not the UBS/ABS functionality. I will try another few rounds of bleed and cycling, and probably loosen a little of the zerk fittings to allow bleed while the pump cycles. (as long as there is some rear brake hydraulic pressure, the cycling can run).