Brakes on my 93 silvarado are acting like prop valve went bad only RT rear works.?

Symptoms started with a RT front caliper intermittently hung up a couple times, then got more persistent. Changed it, got a bunch of air (and sore some moisture likely due to cold rainy weather) in the system, (my teens did that) bled it like 20 times, could not get the mushy brake like no pedal fluid pressure) feedback to go away and its been same since. Changed the master cyl, to affect, then the booster, the FRT RT flex line, LH FRT caliper, no change. Bled the bleeder on the prop valve and the 2 wheel anti lock system several times. The fluid ports to the reservoir like the proportioning valve is byping the front brakes completely and the LH rear. Only the rear RH will work and only completely to the floor with the pedal. The last time I had any problems with a proportioning valve was on a '74 Nova many moons ago. Way to long to remember and much simpler system. Can I take this apart and fix it? They are expensive and I can not shotgun this any longer. I saw a answer that stated I could. I am a technician with lots of tools. Just right now have a wife recovering from surgery and am under a lot of pressure. A girl talking on a cell totaled our van and this is our only backup vehicle big enough for the 6 of us.