Rear brakes not working - maybe the proportioning valve?

[Lugnutz65]

I can't get my rear drum brakes to work.

I must say how much I hate doing the brakes, brake lines and adjustments. Give me anything else, anything! Just not brakes.

I just installed a new CPP front disc brake kit with new spindles, rotors, etc. in my 1965 C10.
I have the Captainfab bracket (nice item) and a salvage yard 1995 Blazer booster and MC complete with the proportioning valve.

I did my research before choosing my booster and MC. The 1995 Blazer has front disc and rear drums. The Blazer front hard lines are 3/16". The rear brake line is 1/4" all the way back to the rear axle where it then reduces to 3/16" to the rear drums.

I bench bled the MC. Tonight I finished my hard lines and have no leaks. Front discs work great. I have nice pedal pressure. The rear lines don't bleed very well but I do get some flow. I just don't get any rear brakes. I adjusted the rear brake shoes. Still nothing.

I figure the rear brake cylinders are NOT getting pressure. So it's either a bad MC or possibly the proportioning valve? Any suggestions before I go spend money on something I don't need?