The only correct way to fit the extension is not to over tighten it and snap off the filler neck! Seriously, there is no way fitting the extension would make any diff with bleeding.
I would suggest - remove all 3 pipes from the brass 3-way union on the firewall. screw in bleed valves in all 3 outlets, then see if you can just bleed the MC using one of the valves obviously and see if you can get pedal pressure. This will eliminate the MC from the equation. Then reconnect the 3 pipes in turn and repeat bleeding process for each brake line. If you have a booster, you can do it more easily by substituting a bleed valve at the outlet and check for air in both the booster & MC.
process of elimination.
I assume you did not remove the MC or disturb anything else.
there must be air in the system somewhere
