This might have been done already, so please excuse me if it has. This might help someone else with the same issues. You said you rebuilt the calipers with new seals, but didn’t mention new pistons or surface condition of both pistons and calipers. I’ve seen corrosion not addressed and new pads can cause binding. This is just because the pistons are now further in the caliper bore. You can put a piece of plywood into the calipers, compensate for pad and rotor thickness. Remove the pads, insert plywood, and squeeze. The pistons should all have identical depressions in the wood. If they are not, then chasing an air bubble is futile. To make sure you got the banjo at the MC cleared, and to find a rogue bubble, zip tie your brake lever so it’s just before applying brake pressure. It needs to be in that dead spot. Leave it on over night. Bubbles in the lines, or banjo should work their way into the reservoir with this method. The best way I’ve found to figure out if lever collapse is air, a bad MC seal, or piston binding, is when you feel that fade, let off and back on the brakes, pumping them. If it’s still soft, then your issue is not air. Good luck and let us know what you find!