Cable tie on the lever overnight causes the fluid to aborb any small air bubbles in it left over from not bleeding properly.
It's only a temporary fix. Picture a bottle of soft drink when you release the pressure by opening the cap. The gas gets out. Happens in blood too - go diving depressurise too fast, the bends. Bubbles of gas escape from the softdrink / blood / brakefluid when the pressure is gone.
If it doesn't straight away with the pressure release (releasing the lever) it will over time just like softdrink eventually goes flat. Heat will speed it up too.