You have the horrible leading-trailing shoe rear brakes. Crappy pedal is epidemic with them.
Verify that the rear brakes are properly adjusted. They adjust by using the park brake, and nobody uses the park brake. Assure that all the usual stuff is still properly functional--wheel cylinder not leaking fluid out, or air in. Shoes in reasonable condition, drums not scored, rear axle shafts 'n' bearings not wiped-out, etc.
Verify that the rotor/hubs up front have properly-adjusted wheel bearings, and the calipers aren't sticking on the guide pins, and the caliper pistons aren't partially-seized.
I'd tell you to assure that the pressure-relief valve in the master cylinder that controls pressure in the Quick Take-Up portion is not leaking or weak...but I have no idea how to do that. I guess you're just supposed to replace the master cylinder.
Bleed the ABS with a scan tool, then bleed the rest of the brake system. See attached.
The REAL fix for the craptastic rear brake problem is to scrap the rear axle. Move up to a C2500 6-lug that has the decent/wonderful 11.x Duo Servo brakes instead of the horse-spit 254mm (10") Leading-Trailing shoe drums. But then you need to deal with the 5-lug vs. 6-lug issues.
Replacing the leading-trailing shoe brakes on my '88 made MORE difference than upgrading the front rotors/calipers, and master/booster from JN3 to JN5.