Try checking for debris inside your tires. If the back is going flat then something is puncturing your tubes. Run your fingers along the inside of your tires and make sure there isn't anything that has punctured through the tire that is puncturing the tube. Do the same for the rim too.
To make things easier pump the tubes that have flatted and submerge them in a bucket of water. Look for air bubbles and determine where in the tires they were located. The culprit/s will be around there somewhere.