I'd strip the calipers down again and check that the pads are in square and true with no debris in the mounting slots, also inspect the caliper mounting faces for any dents, burrs, or debris which would cause them to be bolted up out of true, also remove the disc and do the same for its mounting face on the upright and inside the disc.
If you still have an issue then it could be a flexible brake line has partially collapsed and is producing a one way valve, this can't always be seen from the outside but you can test to see if there's any residual pressure in that caliper by cracking off the bleed nipple after stamping on the brake a few times
Another possibliity is that one caliper is sticking slightly so the piston isn't retracting on its seals
in which case that caliper will need rebuilding with new seals or replacing
One other thing to try is to remove the master cylinder again and have your assistant push down on the brake pedal to expose the pushrod as it exits the servo
on the end of the pushrod there should be a little adjustment nut
try screwing this in a turn or two and put it all back together
this adjustment takes out free play at the pedal so its better to have a little bit of play rather than too tight