+1 on completing a full brake fluid flush too.
Are your brake pads or rotors within acceptable tolerances according to the service manual? Maybe the pistons have been pushed out too far due to excessively worn pads/rotors.
Are you purchasing the calipers from the same place every time? Have you tried an OEM replacement rather than a AutoZone/Advance Auto Parts rebuild unit?
... Something new to me in two weeks ...
'13 Prius Five