96 K3500 dually Soft Brake Pedal

I have a 96 K3500 Dually, 60,000 miles I have replaced the following;
1) All of the brake lines ceramic coated steel, Front to back
2) Rear Brakes
A) Drums,
B) 3.5" shoes 12K, Raybestos
C) Spring kit,
D) Brake Cylinders, AC Delco
3) Front Breaks
A) Shoes, Raybestos
B) Calipers, AC Delco
4) Master Cylinder

Brake Bled, Vacuum Bled, Pressure bled


Brakes stop truck but id constant pressure is apply pedal goes to floor pumping them makes no difference. Let off pedal returns on it own and hard again until constant pressure, real bear at traffic light !!

I have only the brake boost or the ABS Brake system to replace.

Brake were good until I broke a front brake line
Any assistance or ideas would be greatly appreciated

Sounds like a bad master cylinder if it's leaking down under pressure.
I know you said you replaced it but could be you got a bad one.

The issue was found to be the rubber hoses connected to the calipers. Once replaced. Now the brakes are firm !