You shouldn't need the adjustable valve if your combination valve is functioning and you still have rear drum brakes. I don't remember the year, but probably in the early to mid 70's they eliminated the residual valve and went to cup expanders in the wheel cylinders. If you look at old quality wheel cylinders, they have the expanders on the ends of the springs between the cups. They allow the cup seals to stay in contact with the bore of the wheel cylinders so that when the brakes are released air isn't drawn into the wheel cylinders. The new crappy Chinese wheel cylinders eliminated the cup expanders and just make the ends of the springs larger and the usually don't work. That's why after the wheel cylinders are replaced, the brake pedal slowly gets worse because air is being drawn in every time the brakes are released. You'll know if it's the garbage wheel cylinders if bleeding only the rear brakes makes the brake pedal feel normal. I always inspect the new wheel cylinders and if the spring ends look like a 10 year old Chinese kid made them, I use the OEM springs that actually have an expander pressed on to the ends of the spring from the original cylinder.
That's the problem- there is NO combination valve. Just a Home Dupa grade splitter on the front lines. Then that valve on the rear line.
As long as the adjustable valve works, try to adjust it. IIRC on my Wilwood valve, it said to start with the valve all the way out and stop hard from 25MPH. Keep adjusting the valve in 1/4 turn at a time until the rear brakes lock up under a hard stop. Then back it off 1/2 turn. If you know what brand the valve is, go on Summit's website and download the instructions. Make sure that the rear brakes are bled and adjusted before attempting to adjust the valve. If everything seems fine and after driving it for a few weeks the brake pedal seems low, bleed the rear brakes. If the pedal gets better after bleeding the rear brakes, look for better wheel cylinders which is a crap shoot now days. If I get time and can find some old wheel cylinder springs, I'll post some pictures of the different springs between the OE style and the Chinese specials. I just reread your post and if you have an adjustable proportioning valve in the front brake line, it needs to be in the rear brake line, not the front.
