I have a 90 chevy 454SS with poor brake petal.
Beginning problem was with rear brakes to which I replaced the rear wheel cylinders
and rebuilt the rear drum brakes (replaced all parts).
After bleeding the brake system I could get very little brake petal.
Spongey and soft and petal almost to the floor.
Bleed brakes again - no help.
Replaced the master cylinder upon auto shop's advise. - no help.
Capped the brake lines at the master cylinder and found a normal brake petal and no loss
of petal. Reinstalled the back brake line to the master cylinder and had a normal brake
petal. Reinstalled the front brake line and the petal goes to the floor.
I have replaced all of the brake system parts except for the steel brake lines, booster, front
calipers.
Two auto shops, five mechanics, b-w brakes helpline, all my friends and no good results.
Any suggestions are welcome. ????

Welcome to the site Lanternjaw. Sounds like a pressure loss somewhere in the system or air being compressed instead of brake fluid. After the bleeding of the brakes did your problem become more or less noticable? I remember having some air in my brake system causing a very spongy pedal. Very scary and dangerous when braking. Try checking fluid levels are topped up with proper DOT level. Verify that your not losing fluid anywhere in the brake system. Verify that the proper bleeding method has been done with a service manual for your vehicle. Haynes books are pretty good. Any caliper leaks? It's very easy to get air in your brake system but sometimes hard to get it out and be sure it's all out. You've covered a lot of the bases already. Let me know how it goes.

Thanks for the feedback.
I do not have a fluid leak anywhere.
The petal goes nearly to the floor but the brakes stop the vehicle.
the petal does not feel spongey like air in the lines it just travels too far towards the floor.
The brake pads are wearing evenly and the truck does not pull when applying the brakes.
The caliper bolts are clean and not sticking.
Have been thru the haynes manual, and online bleeding procedures.
I appreciate your input.

You may be able to adjust the relationship between the pedal and the master cylinder to make the pedal sit higher so that when it is fully engaged it doesn't hit the floor or close to it. It depends whether or not the master cylinder has adjustment nuts on it. It almost seems as though the pedal or master cylinder have been changed at some time or multiple times and not properly adjusted(connected). I understand you have already replaced the master cylinder with no luck.

This link will show you what I mean.

PART 7 - Adjusting the Master Cylinder pushrod

Thanks again for the info.
I will check the adjustment on the booster rod.
The auto shop guys installed the last master cylinder and I can't for the life of me
rember if they adjusted the rod when they installed it.

lanternjaw, welcome to the site. I hope some of your issues got resolved.