THE OUTCRY that followed enactment of spe cial tax breaks for members of Congress seems to have prompted a reconsideration by the beneficiaries. Good. Just before Christmas, the law was changed so that each member of Congress could take an automatic deduction for living expenses incurred in Washington: an unchallenged $75 for any day Congress is in session and $75-plus if a member offers proof of his expenses to the IRS. While members claimed the new law simply put them in the same position as any traveling businessman, they had, in fact, given themselves a better deal. No one else has automatic deductions or writes off the expenses of maintaining his principal residence.

This week, the Senate Appropriations Committee adopted an amendment doing away with the unchallenged $75-a-day deduction and leaving members with the right to deduct all expenses that can be documented. So far so good. But if they honestly want to say they are following the rules that apply to the ordinary citizen, a further change must be made. Sen. Russell Long knows what it is.

The law now provides that a member's principal place of business, for federal tax purposes, is in his district. This means that all expenses in the Washington area are deductible--even when the member's family has moved to Washington, when he spends almost all his time here and when his home- state residence is little more than a mail drop. Eliminating the presumption of residence, as Sen. Long proposes, would mean that the IRS would apply the same test to a member of Congress that it applies to an ordinary businessman in determining when he is at home and when he is away on business: the amount of time spent in each place, the location of family, and so on. Sen. Long estimates that, if this standard were used, "Washington would be seen as the principal post of duty or place of business for many members of Congress." All expenses of living in or traveling to their home states would then be deductible. If the senator is correct, these deductions would be smaller than those now available for Washington expenses.

Before last December, members of Congress could deduct only $3,000 a year for business expenses involved in maintaining two homes. This figure, on the books since the early '50s, was clearly inadequate. All actual business expenses should be deductible, but not the expenses of maintaining what is, in fact, a principal residence. Sen. Long's proposal points Congress toward equity with other taxpayers, and away from special privileges.