The mortgage industry’s lobbying efforts have succeeded in persuading regulators to extend the comment period on a proposed risk-retention rule for issuers of mortgages. The comment period, which originally was set to expire June 10, will now end Aug.1. Sheila Bair, the current chairman of the Federal Deposit Insurance Corp., has been pressing for a strict rule, but observers are now hoping that since the comment period ends after she steps down, her successor, Martin Gruenberg, will be more sympathetic toward a broader definition.