Who has a KH sbp "non-power" disc in front

Disc brake master cylinders are not the same as drum brake master cylinders. The disc brake master cylinder has reservoirs of unequal size; the reservoir nearest the firewall, which serves the front brakes, is larger. This is because as the disc pads wear, the fluid volume in the calipers increases as the pistons' resting position moves closer to the discs. So a larger volume of fluid is required in the master cylinder to make sure of adequate system volume. There's no such change in fluid volume at the wheels as the drum pads wear, because of the self-adjustors.

Another thing to be aware of: Master cylinders through '72 have a residual pressure valve behind the tube seat of the forward reservoir (serving the rear drum brakes). This keeps low pressure of about 5 to 10 pounds on the line to prevent air ingress past the wheel cylinder seals. The RP valve was deleted starting in '73, made possible by improved wheel cylinder seals. With the improved wheel cylinder seals, an RP valve is optional. Without the improved wheel cylinder seals, an RP valve is mandatory. You mustn't have an RP valve in the line feeding the disc brakes, or they will be always applied even with your foot off the pedal.