Brake Bleeding -Why does pedal get soft after time?

[WB72]

There are a ton of bleeding posts. I have checked most of them, but my problem is more of a question of why is this happening rather than how to bleed brakes.

While I was rebuilding my brake system, I replaced my MC. Bench bled it prior to install.
Replaced all hard lines and rubber lines.
Replaced wheel cylinders and callipers.
I used a vacuum pump to bleed system, then found a bad flare to the rear so I repaired it and re bled lines.
I had a stiff pedal and I left them alone for a month or so.

Today I went to work on it again and pressed the pedal and it was soft and brake light went on. I know it is PV with the light, but what would cause the soft pedal with no visible leaks at all connections?

I am going to have to bleed them again, but want to fix problem at this time before I take the truck on the road for it’s maiden voyage.