We have had a problem with our drains backing up and overflowing in the back bathroom. We ran a snake through all the plumbing and have no obstructions. Someone told a family member that when a newer 1.6 gal toilet is installed in replacement of an older toilet that it sometimes causes back ups. We installed a 1.6 gal in place of an older one with a larger capacity just 2 days before we started having this problem. Any suggestions?