Copper can work harden and not seal. Either replace the copper washer with a new one or heat the defect free one you have to red and let it cool on its own and try again.
Did you peel back the boots and insure no fluid was leaking at either piston cap. Rebuilt wheel cylinder can have pitting and or have been honed oversized incorrectly. Lots of little things can go wrong and all it takes is one.
