The Reagan administration, in yet another break with the policies of its predecessor, will ask Congress this week to lift the prohibition against sales of military equipment to Argentina, according to informed sources.

The request to end the 3-year-old ban will coincide with the visit to the United States this week of Argentina's president-elect, Lt. Gen. Roberto Viola. Viola is to meet here with President Reagan and other top U.S. officials.

Viola's visit and the request to Congress are both indications of the new administration's determination to move away from the human rights emphasis of the Carter administration's foreign policy.

The sales ban was imposed in 1978 under an amendment sponsored by Sen. Edward M. Kennedy (D-Mass.). At the time, Argentine was seeking to purchase about $100 million in military equipment but was under heavy criticism for human rights violations.

Last year, the Carter administration, looking for ways to improve relations with Argentina, considered resuming military aid to that country. But the proposal was dropped in the face of fierce opposition from human rights advocates in the State Department.

The Reagan administration, however, has deliberately set out to downgrade the importance of human rights, and thus its decision to seek lifting of the ban on military sales to Argentina is not surprising.

In announcing the Viola visit, for example, State Department spokesman William J. Dyess noted the recent "abnormality" in U.S.-Argentine relations was due in large part to the Carter administration's outspoken position on human rights. And he promised that future criticism of human rights violations in "authoritarian" but friendly countries such as Argentina would be conducted in private.

The request to lift the military sales ban to Argentina is expected to go to Congress along with a far more controversial request to lift a ban against overt or covert U.S. aid to rebel forces in Angola.

These actions may provide critics of the administration's human-rights policies and its courtship with military dictatorships with a focal point for opposition.

Earlier, the administration announced another controversial step involving a military regime in South American when it said it was lifting two of the sanctions imposed on Chile for its refusal to cooperate in the investigation of the role of some of its ex-intelligence officers in the murder here of former Chilean diplomat Orlando Letelier.

Sources said a review of a similar ban on military ales to Chile was also under review in the administration, but it could not be learned whether a request to lift the Chile ban would be part of the same package.

Such a request would appear to contradict the testimony of John A. Bushnell, the acting assistant secretary for inter-American affairs, who last weel told a House subcommittee he did not anticipate any change this year in the legislative sanctions imposed on Chile in the aftermath of the Letelier murder.