You aren't receiving equal flow front/rear. The mc deals with that.
The prop valve bleeds pressure away from the rear of the vehicle when it gets light so it doesn't skid.
79-83 trucks didn't use the T fitting at the PF like the later stuff did.
If you have weak front brakes, you need to...
Imo eliminating the lspv is the answer, especially with big rubber.
If your front axles doesn't already have the vented rotor/ifs caliper update, do it too.
I rec the v6 dual diaphragm booster and MC.
I also run the "big brake" kit. (fror or sord)
I can lock up my 37s no issues