Just checking your steps, found this

https://howtune.com/articles/178-ble...sler-crossfire

Positive, all connections are tight ?

I have not done my crossifre slave cylinder, but when bleeding, I wrap a loop in the hose before i drain into a bottle,, the loop helps with keeping fluid in the line and less chance of sucking air back in, sometimes also if i have short enough hose, i run to bottle with fluid and if it tries to draw back, it sucks in oil which gives alittle resistance.
some greas on threads helps sometimes also from air pulling it from the loose threads.