Slowly reviving my 68Xr7. While my brakes work, they don’t seem to work well as I feel they should. I have a new master cylinder, new soft brake lines and a rebuilt power booster. It’s my feeling, that the rear brakes may not be contributing their fair share, although I have adjusted them. Is it possible the proportioning valve is not working correctly? Prior to my ownership this car sat for 25± years.
It is a typical thing to swap the disc brakes left to right. The bleeder ports face to the rear if they are correct. If the ports face upward you have a lot of trapped air that cannot be bled.
The proportioning valve sometimes sticks open which will mean the rear brakes get most of the braking. That does not sound like your problem. The other thing proportioning valves like to do is leak. Generally that means they work fine until you are out of fluid. Again does not sound like your symptoms.
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Our 67 did the same thing til i found a brake and clutch reline shop…slightly softer rear shoes and re
arc the shoes to new drums . Most automotive machine shops will not arc the shoes.
What brand (quality) of brake shoes are you using? I always used the brand from the local auto parts stores, like Carquest from Advanced Auto. I also didn’t like the way my brakes felt. Eventually I switched to Porterfield and felt that it was an improvement.
Couldn’t tell ya. It’s whatever prior owners put on more than 25 years ago. 
I’m guessing it’s master cylinder related. Are you certain you got the brake pushrod set to the correct length? They make a tool for this. Also, does your master cylinder have a residual pressure valve for the rear circuit? If not you’ll have long pedal travel.
I’m leaning that direction as well. I bought a Brake Booster Rod Adj tool to check it. That is my next step. My clearance may be off.
Take a photo of your front disc calipers.
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Im still going with rear shoes
Take off drums and wipe shoes with lacquer thinner, then mark the shoe with a sharpie
Reinstall drums and drive 5 or so miles normally. Remove drums . Id be surprised if the shoes are hitting more than 3 inches in the center.
Looks right. Still might be bleeding technique.
I found the bleeding on my 68 disc & drum system very difficult after a near complete rebuild. I torque the bleeders considerably more then ‘spec’. Lesson learned. In the end I did the two person bleeding technique but also had a MighyVac pulling 15psi of vacuum. Basically pushing and pulling on the hydraulic as I opened the bleeders slightly.