Rates on 30-year mortgages fell this week to the lowest level on record after the Federal Reserve launched a new effort to assist the staggering U.S. housing market.Mortgage finance giant Freddie Mac said Thursday that average rates on 30-year fixed-rate mortgages dropped to 4.85 percent this week, from 4.98 percent last week. It was the lowest in the history of Freddie Mac’s survey, which dates back to 1971 and was down a full percentage point from a year ago.The previous record low of 4.96 percent was set in the week of Jan. 15. Rates fell after the Fed last week said it will pump $1.2 trillion into the economy in an effort to lower rates on mortgages and loosen credit.Rates on 30-year mortgages traditionally track yields on long-term government debt.Though the yield on the benchmark 10-year Treasury note initially plunged by about 0.5 percentage points after the Fed’s move, lenders did not pass the entire drop on to borrowers. Bond yields rose after worries about what some saw as lackluster demand at a government debt auction Wednesday.