The Federal Home Loan Bank Board, in an effort to prop up the nation's ailing thrift institutions, reduced its liquidity requirements yesterday for savings and loan associations.

With net savings in February at a 10-year-low, Bank Board Chairman Jay Janis cut from 5 1/2 percent to 5 percent the amount of savings and short-term investments that thrift institutions must keep on hand to pay withdrawals.

In addtional, the board reduced the short-term liquidity requirement from 1 1/2 percent to one percent. Both actions are effective April 1.

Although the move will free up as much as $2.3 billion for S&Ls, very little of the money is expected to find its way into the mortgage market. The funds probably will be used instead to prop up ailing thrift institutions, which are putting most of their money into money market instruments to get more return on their dollars than they pay out in interest.

Next week, the interest rate on six-month, $10,000-minimum money market certificates will climb to 15.7 percent. Some S&Ls now have as much as 40 percent of their deposits in these and in $100,000 jumbo certificates on which they pay 17 percent or more interest. Sales of 2 1/2-year certificates slowed after a 12 percent cap was put on them.

Indeed, net new savings funds dropped 61 percent in February compared with the previous year. The $1 billion inflow was the poorest since February 1970. Noting the entire gain occurred during the first part of the month before the sharp rise in interest rates caused by the Fed's credit tightening, Janis saw this end-of-month decrease as a portent of poorer savings flows in coming months.