New Master Cylinder and can't get air out of one brake line

first let me say that I don't have a ton of experience working on brakes so I may be doing something wrong. Working on a 71 Dart with manual drum brakes all the way around. Has new shoes, wheel cylinders, ect within the last year.

Recently realized my master cylinder was leaking so I purchased a new aluminum 15/16 master cylinder from Cass at Dr Diff. Didn't have a good way to bench bleed the new MC so I installed on the car and bled the MC on the car by cracking the lines and having wife pump brakes. Got all air out of MC and the rear brakes bled as expected. Had a very difficult time getting the air out of the front passenger side line but finally did. Now matter how many times I try I can't seem to get all the air out of the front drivers side.

I have made sure to keep the MC full of fluid while bleeding. Can't find any leaking lines. Any ideas? Wanted to see if I'm missing something before I call Cass and ask him. Thanks in advance for any help.