I have a expansion tank that overfills, I have manually adjusted the float valve i.e. bent the metal arm. Adjusted the height with the shut off valve and taken out water to reduce to new level yet it fills to 2/3 of way up the float and then overflows. Any suggestions as their does not appear to be a leak from the valve and it appears to shut off the water?

What kind of valve is it. And if you don't know try to describe the looks of the valve.

I would take the float valve apart and check the washer and seat. The washer to see i8f it's bad and needs to be replaced and the seat for any debris that may be preventing the washer from sealing when it shuts off. Good luck, Tom