First time having manual brakes, are mine bad?

Dale Davies said
One thing to check is the master cylinder bore diameter. Drum brake cars had smaller diameter wheel cylinders which required less fluid as the pistons traveled. This required a smaller diameter master cylinder to develop the necessary braking force. The disc brake caliper cylinders are larger diameter and take more fluid to clamp the disc. The master cylinders with the same bore as drum brakes would require longer travel to get braking pressure. The answer to the long travel was power assist by vaccum booster, and a larger diameter master cylinder to get the required pressure. If someone in the past has asked a parts store for a disc brake master cylinder and bolted it in, you will have the situation you describe. How is pedal travel? A disc brake without booster should have an intermediate diameter between the manual drum and power disc cylinders. Remember that these model years were when disc brakes were being introduced along with power assist, so many parts were changed.
Another point, where the brake lines thread into the master cylinder there are residual valves. On drum brakes these hold about 10PSI in the lines to ensure no air can get in. Drum brakes have heavy return springs so the 10PSI can not hold the shoes on the drum. Discs use runout or the squ
are piston seals to retract the pistons slightly, so only 2PSI residual valves are used. All drum vehicles will use two 10# residual valves, 4 wheel discs use two 2# residual valves while disc/drum cars use one of each.
Dealing with parts counters these days is a trip to the lounge for a couple of whiskys experience. Even the old dogs behind the counter have a rough time because the old parts books have been swept away for a glowing box full of stupidity. In those books was a wealth of information where you could quickly check master cylinder diameters for example and compare. The glowing box requires make, model, year, engine and probably transmission, rear axle size and I usually tell them the color. They may require the date your wife washed your underwear by now. To find out about different cylinder diameters is near impossible. Closest you get is drum or disc brakes. The concept of manual discs is beyond their comprehension, but they were manufactured and require a different master cylinder.

So let's get theory and reality lined up here. There were only 3 different sized master cylinder bores for A-bodies from '67-76- 15/16", 1", and 1-1/32". That's it.

Stock A-body master cylinders were 15/16" for 9" drums, 1-1/32" for 10" drums, 1-1/32" for manual single piston (73+) disks, 15/16" for power 73+ disks, and 1" for the KH disks. From the '73 service manual, "L" is the A-body designation. The 2.75" calipers were B/E/R/F/M/J body fare, pin style in '73 hence the "floating" designation, the 2.6's were A-body single piston slider calipers. Since this chart is from a '73 manual it doesn't include the '76 A-body 2.75" piston sliders, but they used the same MC as the 2.6" calipers on A-bodies anyway.

From the '70 service manual, for the fixed caliper KH's used on A's

As you can see, A-bodies are actually backward when it comes to manual and power disk master cylinder bore sizing. That's because the power booster linkage changes the pedal ratio. So the power disk cars actually got a smaller bore master cylinder than the manual disk cars.

I like the 15/16" bore for manual disks. I actually think the 15/16" master cylinder improves your brake modulation capabilities quite a bit with the slightly longer travel, but it still provides the max line pressure. And the travel is not "too long", there's still plenty of space before the pedal gets anywhere near the floor.

Dale Davies said
So what residual pressure do you have in your 4 wheel discs?

None is required. The head pressure from the height of the master cylinder is enough. If it were an early truck with the master cylinder under the floor or something you'd need the 2 psi residual. Most disk master cylinders don't have any residual at all, they have to be added.