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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
do you still need help?
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