I used to have to add +1.5mm to the brass ones after they get caked with minerals, making them 27.0-27.5. The real problem isn't with the floats, but that the mass of the floats, especially when dirty, causes slight gooves to develop in the little posts where the pivot rod is supported. These little grooves tend to make the floats stick when the bike is sidestanded, particularly.
A tiny file will smooth off those ridges (look close to find them, don't file much), and smooth off the now-sharp edges of the rotating float "bearing", as well. Then, you might get away with the 26mm setting again. And, make sure they haven't accumulated any internal fluid. They sometimes would condense water inside, just a drop or so. You can hear it splash if you shake it in a real quiet room and listen close, then you have to drill a tiny hole, heat the float (like on an iron) for about 15-20 minutes, then cool it and resolder. That's the only way I could reliably dry the brass ones out.
