Like I said, we bought a 1.3 gallon toilet and modified it to "old
style". The toilet tank of our 20 year old "normal" toilet is EXACTLY
the same size as the tank on our 1.3 gallon toilet. The only difference
was that the new toilet had a 1.3 gallon plastic tub inside the tank
which was all that would fill with water. We removed the tub, got a new
float, and no more problems. Sorry yours isn't like that. Perhaps
because the manufacturer decided it was easier/cheaper to add the tub
than to re-tool the factory. Who knows?