Rear Brakes Locking Up After Disc Install

[Gator Alum]

I installed the POL kit with front discs, power brake booster, and proportioning valve. I also installed a 2lb residual check valve for front and a 10 pound for the rear. The truck currently has 3/16" line throughout and a single line going to the front that T's off and a single line to the rear that T's off.

The rear brakes have a major tendency to lock up rather quickly under normal braking conditions. The pedal seems soft/light until the bottom and then the brakes grab hard and the rears lock up which tends to wake the neighbors.

Could it be improper drum adjustment in the rear? Maybe improper adjustment of the push rod on the MC?

As always, any help/insight is appreciated.

-Danny