brake issues, out of ideas...

If the pedal changes "a little" when you first move, THAT COULD BE the shoes shifting. But once you are moving, and have made about 1 stop, that nonsense should stop.

There are only two possibilities as I see it:

A leak to the outside, losing fluid. You say this is not the case

Master cylinder bypassing internally. Once again, a new/ rebuild cylinder DOES NOT mean a "good" cylinder

Last possibility I posted above is the brake warning switch leaking internally BUT AFTER RE THINKING that could not be the problem because there is pressure on both sides

CONCLUSION

IF IF IF you are NOT losing fluid externally, the MASTER is bypassing internally