1984 Monte Carlo. When I bought the car it had a jegs rear disc brake conversion on the back(no emergency brake). I can’t seem to get enough fluid pressure to the back calipers for them to shut on rotor. I thought it was the proportioning valve. So I Changed it to a disc/disc valve and still the same. Ordered a new stock master cylinder and still the same. My question is should I be running a different master cylinder for a disc/disc application as well or what exactly do I need to make it right and work properly cause it’s kicking my *** trying to figure it out. Side note. Everything was bled properly and master cylinder as well. Used the proportioning valve tool to keep it locked while bleeding. So hypothetically everything is good but the pressure to the rear calipers to have them close on the rotor tight. The lines are getting fluid enough to bleed rear brakes but no pressure for position to do its job properly. Any help or pointing in right direction would be great. Thanks

 

Just a long shot but what about the Brake booster? Are all the Vac lines to it good or if they are leaking it might hamper this. Or it could be you need to put a new and upgraded Brake booster on the system.
I recall watching one of the fix and restore shows had an issue like this.