I'm sorry, I missed the part about the firewall mounted master cylinder that you put in your original post....disregard the stuff about under floor mounted masters and pedals.
it is easy to tell the difference between a combination valve for disc/disc and one for disc/drum.
The rear outlet looks much the same, but on the disc/drum combination valve between the 2 outlets for the front lines, there is a small screw in fitting with a button that sticks out, usually these are covered with a rubber boot. That is part of your metering valve. The disc/disc combination valve does not have this.
The wrong combination valve would not make your brakes feel bad, or not lock at least the fronts properly.
Residual pressure valves are needed for the rear drum brakes , those are stock in just about all applications. Having 2 valves in the line will not affect the function at all.
If the pedal feels ok, but the brakes don't lock, you probably have air in the lines, or your calipers are pulling back more than they should, or the rear brakes are not adjusted, there are a few other reasons, but start with those.... You can run a 2# RPV in the front line, it won't hurt a bit, it usually is not needed, but there are times when it will help...more on that after you answer a few questions...
Most cars in the weight range of a 38 chevy that use disc/drum use a 1 1/8" bore MC with an 11" booster and a 4:1 pedal ratio. If you lengthened a pedal to fit at the bottom, you changed the pedal ratio to a higher numerical ratio.. If you lengthened it at the top, you lowered the ratio.
Lets forget about valving for a little bit and get some of the basics out of the way.
You have a set of GM calipers in the front, which ones, metric or early calipers?
What is your master cylinder bore size?
What is your measured pedal ratio?
How much travel at the master cylinder pushrod do you have using the full available pedal stroke...?
What size are your rear wheel cylinders?
Actual measurements please..
The disc/disc master cylinder is no different from a disc/drum master cylinder other than it has no RPV installed in the rear line. Drum/drum MCs have 2 RPVs...The range of available bore sizes are the same.
Some GM cars use a step bore master cylinder, but we won't go there just yet.
my signature lines...not really directed at anyone in particular..
BE different....ACT normal.
No one is completely useless..They can always be used as a bad example