Soft brakes pedal sticking?

Hello, is the fluid in the master cylinder still clear? or is it dark now? If its clear, the fluid should be ok. When the master cylinder seals fail the fluid will turn dark. I would start to look very close for any leaks, if there is a leak then air is getting into the system as well. Hence the soft pedal. Since you have all new lines, I would look at the fittings for leaks. Did you have the rear drum shoes replaced with the wheel cylinders in back if they did the whole system?

You have very low mileage for the year, is that the correct reading?

Yes the fluid is clear and also yes the rear drums were completely replaced

That is the mileage for the crate 5.7 and trans which was done at the same time a few years ago

The body has 312k

Ok that fluid looks fine. But it is low. I would top off the Master, and have someone start it up and press the pedal, just to be sure you dont have a small leak somewhere that was missed before. Just so Im clear on things, I assume they replaced all the brakes lines, master, calipers/pads, rear shoes/drums-wheel cylinders on each side,?
Some of the older Chevys had those brake lines running inside the frame rail, they were a pain and most of the time we just ran new lines, front to back instead of inside the frame. If thats the case, keep looking for a leak, start with the rear brake wheel cylinders that push the brake shoes out, even if they are brand new they can leak. We are getting a rash of bad parts lately.

Now with an ABS module, air will get stuck in places that a gravity bleed wont always get. We use a scan tool on the newer vehicles to open up all the ABS solenoids completely to get the air out. I use a brake vacuum bleeder as well if needed. Some newer and even old vehicles can just be very difficult to bleed the air out.
Along with the air bleed, the rear brake shoes may not have adjusted themselves out all the way. Those we had to pull the E brake going in Reverse sometimes to get the shoes to adjust out, I remember that.
Are you confident enough to work on the vehicle? Remember, this is the brake system, not something you want to make a mistake on.
And please let us know what you find so we can solve this, no one wants to see a brake failure.

Here is a guide to help.
https://2carpros.com/articles/brake-pedal-goes-to-the-floor/

And something I should have added, if the master cylinder was replaced, I am pretty sure the brake booster had a push rod that might need adjusting.
You can also check that the booster didnt fail, just unbolt the master cylinder, leave the lines attached and look for any leaking fluid from the brake booster when its unbolted from the master. its only 2 nuts to take off.

If you dont see any fluid leaking anywhere if the fluid level goes down it could be leaking into the brake booster behind the master cyl...but if the fluide level doesnt change then its most likely leaking internally inside the master cyclinder but either way youd need to rebuild it ir just replace it

Thanks for the advice?

Also thats a good point about the pushrid not being adjusted correctly thats a good possibility also
But sound like a priblem that would develop up there In that area for sure
good luck hope this helps

I did not mean to put a question mark there at the end oops

Does this truck have ABS? IF so it sounds like there is air in the ABS unit this video will show you how to bleed it.

https://www.youtube.com/watch?v=nQRnKyVyMuQ