I'm thinking that you have it set up as close to right as you can get it.
I had my wife'd mini van do something similar a few years ago after I changed a steel brake line and found out that one of the flares on the line wasn't sealing perfectly and was letting air back into the system but not showing a fluid leak. We would bleed the brakes she would drive it to work for about two days and we had to bleed the brakes again. I pulled the line off my little Ram panel van and swapped them and that fixed it. Especially if it seems to be happening at one end you might consider checking the flare fittings.
The only other thing I can think of is that it isn't that hard or expensive to make a pressure bleeder using a one gallon weed sprayer
This one is pretty simple
http://faculty.ccp.edu/faculty/dreed...eder/index.htm
You could make a plate for the top of the Mc with a piece of steel or aluminum plate that was large enough to cover the top of the master cylinder and tap it for a fitting to hook to the hose and make a gasket out of a piece of rubber. You would have to figure out how to clamp it on the master cylinder but maybe a modified cheap C clamp would work there. That would let you push any and all air out of the system.