OOPS - I should have said pass. front....
SO, the one not working right IS the one you replaced....
At this point in a PITA, if it was in front of me I'd start re-checking everything to rule out any "simple" or "stupid' problem before giving up & replacing the master cyl.. (As one prev. poster mentioned, they CAN give up from the exercise of bleeding them but that is more common for older cars)
Leak at any connection disturbed in the replacement process.
Replacement caliper "jamming up" in some manner so it flexes instead of pumping up properly to press on the pads. Look at it when someone steps on the pedal, this issue is usually obvious when you can watch it working, you'll see it flex & return to it's prev. position instead of only moving SLIGHTLY as pressure is applied. Pads can be "stuck", calipers can not move right on their pins, the list goes on....
Next "odd" one I've seen SHOULD be "obvious", but I HAVE seen it happen. The new caliper COULD be the wrong one! Believe it or not, I HAVE seen a "mirror image" situation where the new caliper was for the wrong side, and when bolted in place the inlet was on top & the bleeder screw on the bottom. This makes it virtually impossible to bleed ALL the air out of the new caliper and is ACTUALLY the most common cause for the issue you are dealing with. It bolts up fine, but just doesn't work right! And in looking for a deeper issue, the "obvious" easily gets missed.
Lastly, buying or borrowing a vacuum or pressure bleeding tool can help in making sure the system is completely bled of air.
Good Luck, let us know how it all turns out!