Need help w/ master cylinder

[68jimmy]

Well I'm almost done with my disc brake conversion and have run into one more snag... Mr Murphy has been hard at work in my garage

The power booster master cylinder and prop valve were taken off of a 72 parts truck I have. The bleeding of the front brake went ok with no issues. Now when I started to bleed the rears it just seemed that little to no fluid was moving. So I went to the master cylinder and tried to bleed from the fitting just above the hard line that goes to the prop valve. After I had my wife pump up the brakes and I crack open the bleeder what come out is frothy and not much fluid. And the pedal does not go down... When I do the same with the bleeder for the front brakes it I get plenty of fluid with no bubbles. I then removed the hard line from the mc to the prop valve and it's still the same. Do I need a new mc? Or should I check something else?

In the attached pictures the hard lines for the brakes are as follows: The left on the mc feed the front brakes and the right line to the rears...

Thanks in advance!