Disc Brake Conversion Failing

Hey Fabo Fam's, it's been a long time and A LOT has been done to the Barricuda. I will post plenty of pics of whats been done and also post all of the parts that are up for grabs. But I do have a question for anyone that can help. The car is 67, I did a 68 disc brake conversion up front. the brakes were stopping but not locking and really doesn't feel much different from drum/drum application and the pedal does feel pressure. I replaced the drum/drum MC with a disc/drum MC and also replaced the stock distribution valve for what I think would be the correct proprtioning valve. Welp after all of this only one wheel is locking and it happens to a rear one at that. All of the wheel come to slow stop except the passenger rear. Pleeeeaaase help. MC was bench bleed, no air in the system (I've bleed and bleed the system) new shoes in the rear, new pads up front, new everything and no locking. Any suggestions?
Had similar issues, the front brake line at the master has a residual pressure relief valve, make sure it has been removed for your front disc brakes, the valve is for disc brakes, even though I had replaced my master with disc front rear drum the remanufactured mac still had a residual valve, I cant recall who on here brought it to my attention, but I removed front line, screwed a sheet rock screw in and pulled the residual valve spring out, tah dah, fixed, when I tell you many dollars in new masters/ calipers/portioning valve and brake lines to get to a simple fix. Check it out, you may be pleasantly suprused!