Although the runaway growth of the Social Security disability program has slowed down substantially, no "rollback" of scheduled Social Security tax increases is justified yet, the system's former chief actuary told Congress yesterday.

Robert J. Myers, chief actuary from 1947 to 1970 and now a professor and consultant, also criticized President Carter's plan to put a special ceiling on family disability benefits to discourage malingering.

While the Carter plan seeks to reduce benefits to a level where they aren't a disincentive to rehabilitation, it hurts the lowest-income families the most without cutting back benefits to the higher-income ones, Myers said.

For example, according to Myers, under existing law, if a $4,000-a-year worker with a wife and child becomes disabled, his family monthly disability benefit is equal to 89 percent of his previous take-home pay. Carter would cut this to 73 percent.

But for a $17,700-a-year worker the benefit would be 75 percent -- no reduction at all from the present level. Myers called this "very, very bad" and "the reverse of social justice."

He told the House Social Security subcommittee it would be better to cut the lowest-paid worker only a small amount -- to 83 percent -- while scaling the higher-paid ones down a bit more, dropping the $17,700 man to 65 percent.

Myers, questioned by subcommittee chairman J. J. Pickle (D-Tex.), said the Social Security disability trust fund -- which pays benefits to workers below retirement age who become too disabled for any job -- is now in good shape.

Part of the reason is the schedule of tax increases voted in 1977, he said, and another part is a downturn in the explosive growth of the program, which jumped from about $6 billion in 1973 to about $15 billion in 1979.

He attributed the downturn to improved administration and the end of the recession.

But he warned that the reduction in the number of new awards each year, which is down markedly only in the past year, hasn't lasted long enough to be counted on permanently.

Moreover, he said, any savings resulting from the reduction could easily be eaten up by inflation, necessitating higher annual cost-of-living increases in benefits than are now foreseen.

Therefore, he said, he wouldn't favor rolling back Social Security taxes, which are now 6.13 percent for employers and employes but will jump to 6.65 percent in 1981, merely because "the disability experience now seems much more favorable than in the past."

Pickle, looking at the statistics, said the slowup in growth of the disability program is "reassuring but not exactly elating." He agreed with Myers that "it doesn't mean we're going to have a surplus on hand."

He said he would ask Health, Education and Welfare Secretary Joseph A. Califano Jr. why he hasn't sought about $40 million needed by the states to continue closer medical examination of claimants and other administrative improvements -- the ones many believe help explain the reduction in new awards.