The House rejected an emotional plea from Rep. Claude Pepper (D-Fla.) and voted 235 to 162 yesterday to slash Social Security disability benefits by about 15 percent for families of disabled workers coming on the rolls after Jan. 1, 1980. Those already on would not be cut.

"Are we so destitute in America, are we so hard-pressed," that in order to save an estimated $1.1 billion a year in benefit payments by fiscal 1984 "we would take it from the crippled . . . the children of disabled people?" shouted Pepper to applause from a handful of colleagues.

However, Rep. Al Ullman (D-Ore.), Jake Pickle (D-Tex.), bill sponsor, Barber Conable (R-N.Y.) and others said the bill is "a very modest reform" that will slightly cut maximum benefits and make sure a family doesn't end up with benefits actually higher than earnings prior to disability.

Pickle said this is true of 6 percent of families receiving disability benefits.

Critics have aruged that excessively high benefit levels discourage attempts to rehabilitate workers and get them off the rolls.

A coalition of organizations of the aged and disabled led by former Secretary of Health, Education and Welfare Wilbur J. Cohen opposed the bill and managed to keep it from House floor action for nearly five months.

They argued that the changes in future benefits for those coming on the rolls after Jan. 1, 1980, were too harsh, especially in view of the fact that an explosive growth in the program has slowed substantially. At present there are about 4.9 million workers and their families on the rolls, and program outlays are estimated at $13.5 billion a year.

The bill now goes to the Senate Finance Committee.

The two key features of the bill limit total family benefits to 80 percent of the worker's gross earnings before disability, and reduce the number of low-earning "dropout years" that will be disregarded in computing prior earnings. As a result, a worker whose yearly earnings averaged $16,000 prior to disability and who would have received $10,776 in annual benefits under existing law if he had two dependents, would get only $9,238 under the bill.

In addition to benefit reductions for future disabled worker families, the bill improves certain benefit conditions in order to encourage rehabilitation efforts by those seeking to get back to work.

It allows disabled workers to ignore certain work expenses due to disability in computing whether their incomes are too high to permit benefits. It allows them to take a 24-month trial-work period instead of nine months without having to reapply for benefits if the effort to work fails. It eliminates a second six month waiting period for Medicare benefits, which disabled workers get regardless of age, for those who return to the rolls after a work-attempt fails.