Drum brakes with self-adjusters at the bottom (Duo-Servo) may self-adjust when braked firmly in reverse. This WILL NOT WORK on drum brakes with the adjusters at the top (Leading/Trailing shoes).
Drum brakes with adjusters at the top may self-adjust when the park brake is applied and released, then the service brake is applied. The park brake "*****" the adjuster, the service brake "pulls the trigger". Depending on how far the adjustment is from correct, it may take several repetitions. Damned pain in the ass. I happily upgraded my K1500 from the dreaded leading/trailing brakes (adjuster at the top) for 6-lug K2500 duo-servo brakes (and rear axle assembly) with adjuster at the bottom.
With EITHER system, one notch on the adjuster wheel is damned sure NOT the difference between normal brake pedal and pedal-near-the-floor.
If a residual check valve doesn't allow the "shoes to retreat all the way" you've screwed-up the brakes. The shoes MUST retract properly or the self-adjuster won't work right. Residual pressure valves were NOT intended to prevent full retraction of the shoes. They're intended to maintain a slight positive pressure in the wheel cylinders, so air cannot be "sucked" in when the brake pedal is released. Wheel cylinder seals are primarily intended to keep fluid in--not to keep air out. On most vehicles, having the master cylinder mounted a couple of feet above the wheel cylinders is all that's needed. The height difference provides some minimal pressure in the wheel cylinders.
I've also run across guys who adjusted the park brake cable too tight, that won't allow the shoes to retract properly, and therefore also screws-up the self-adjuster. ALWAYS adjust the service brakes before adjusting the park brake.
And while rear brake adjustment is PROBABLY the cause of low brake pedal and the brake light illuminating, it is entirely possible that there's air in the system. Adjust first, re-bleed if the adjustment doesn't fix the low brake/brake light.