Another guy with brake problems

[70c-10]

This is on my 1988 C-1500.

Last week, the parking brake light came on while I was driving. I pulled over and saw that the brake fluid was low, so I added some fluid and the light went out. A day later, the same thing happened, so I looked around and saw that the one of the rear wheel cylinders was leaking pretty bad.

I brought it to a shop to have the cylinder changed and have them just check the brake system over. They found that the other cylinder had a small leak and that the front pads should be changed. After they swapped these out, they said that the pedal felt really soft. Keeping in mind that the trucks has always had a soft pedal, I went to check it out. It was really bad, at that point we decided that maybe the master cylinder was bad, so they swapped that out, but the truck continued to have the same problem. Today they swapped out the the EH / isolation dump valve. I haven't been by to check out the truck, but they said that it is having the same problem as before. They also mentioned that it feel like it has pedal before you start the truck, but once you start it the pedal will drop to the floor.

Any ideas? The next step is to change the rear pads, but it's a desperation move at this point. I have read something about 'flex lines' being a possibility, but it's just weird that the 'soft pedal' problem has gotten worse since swapping in this new stuff.

so is summary, this is what has been changed already:
both rear wheel cylinders
front pads
master cylinder
EV / isolation / dump valve