It's nprmally best to flush through without draining, if you are only changing the fluid.
The problem you get is the airlock you are struggling with.
I find the just flickng the lever tip back and forth while tapping the hose at the same time helps to
get the fluid running down the hose.
another option is one of the pumps to send fluid in from the bleed nipple end, but I have never used one
myself so have no experience of them.