It actually sounds like you may have a malfunctioning, or improper combination valve. Did you receive a new combination valve with your front disc brake kit? If not, did you re-use the factory drum/drum distribution block?
Here's a link with great information on how a combination valve works.
https://mpbrakes.com/combination-val...nation-valves/
Unless you're fine tuning for performance driving, a proportioning valve is almost never needed over a proper combination valve. What it sounds like is happening in your situation is that the hold-off valve is either not part of your combination valve, or isn't functioning. You may also have a displaced shuttle in the combination valve that is partially blocking fluid to the front brakes. I am concerned that you're not able to lock the fronts. you're looking at the rear brakes as having the issue, but I think the fact that you're unable to lock up the front wheels is the more concerning symptom. Fixing that will likely have an effect on the rear.
In your case you want to make sure you have a disc/drum combination valve. Not a distribution block designed for drum/drum applications, or a combination valve designed for disc/disc applications. You need the hold off valve to keep the disc brakes from applying before the drums and causing the nose to dive. That rapidly unloads the rear tires and will cause them to lock prematurely.
If you bled the brakes without a shuttle centering tool installed in the combination valves, it's possibly you tripped the valve. If that's the case you need to remove the valve, recenter the internal shuttle, then rebleed the system with a shuttle center tool installed.
Verify all this first and make sure the system is properly bled.