Which 7/8 master cylinder?

Here ya go, here's why I don't like the front piston going to the front brakes.
If you get the pressure transfer chamber full of air, only the rear brakes will work, and the braking will be poor.
I think you are gonna have to re-bench bleed that puppy.

If that don't cure it, then you will have to take it apart and make sure it was assembled correctly.
If it was my M/C, and it had a big and a little reservoir, I would interconnect the chambers about half way down, and then I would plumb it like a Mopar, IF IT WAS MINE, and then I would check the fluid level more than once a year,lol. I like knowing that the front brakes are directly activated by my foot.