I the RWAL is working, I would continue to use it and just replace the proportioning valve. The only problem was front sensor gap due to rust or wire harness failures due to corrosion. These wires are twisted pairs, 9 times per foot at the sensor and in a harness extension.
Newer models in the GMT800 series with ABS, the proportioning valve is only a balance valve with hold-off pressures to the rear disc are in the ABS line head. The module controls lock-up pressures for the rear using a high resolution VSS signal from the trans and not each rear wheel, or 3-channel which uses a sensor on the diff that you probably have.
You have opened a can or worms here, but the adjustable proportioning valve is your only hope at this point.
Master cylinders for rear drum have a residual line pressure valve of 10 lbs. inserted behind the flare insert seat in front of the line. The OEM proportioning valve contained the hold-off pressure point for the rear disc and pressure differential valve w/spring pre-set for drum or rear disc.
I would use the OEM one for disc/disc, but you have an adjustable one already. It would be installed on the rear line only. Hope this helps.
