Rates on prime mortgages eligible for purchase or guarantee by Freddie Mac hit another new low this week.

For borrowers making 20 percent down payments, the 30-year fixed-rate mortgage averaged 4.78 percent with an average of 0.7 point for the week ending April 2, down from 4.85 percent a week ago and 5.88 percent at the same time last year. The rate has never been lower since Freddie Mac began the survey in 1971.

Rates on 15-year fixed-rate mortgages were also at lows not seen in records dating back to 1991, averaging 4.52 percent with an average 0.7 point, down from 4.58 percent last week and 5.42 percent a year ago.

Five-year Treasury-indexed hybrid adjustable-rate mortgages (ARMs) averaged 4.92 percent this week, with an average 0.7 point, down from 4.96 percent last week and 5.59 percent a year ago. The 5-year ARM has not been lower since the survey began tracking that product in 2005.

One-year Treasury-indexed ARMs averaged 4.75 percent this week with an average 0.6 point, down from 4.85 percent last week and 5.19 percent a year ago. The 1-year ARM has not been lower since the week ending Sept. 29, 2005, when it averaged 4.68 percent.

Low rates have spurred a boom in applications to refinance, but more modest increases in applications for purchase loans. According to the Mortgage Bankers Association, applications for refinance loans were up 141 percent last week from a year ago. Applications for purchase loans have picked up in recent weeks, but were still off 32 percent from a year ago

Purchase-loan applications were essentially flat, but applications for refinance loans were up 3.7 percent for the week ending March 27.  That's not as dramatic as the 41.5 percent increase in refinancing applications seen during the week ending March 20, following the Federal Reserve's announcement that it would expand its balance sheet by $1.15 trillion to keep interest rates down and stimulate borrowing.