Reaction to the death of Leonid Brezhnev by American specialists and political figures, including three former presidents, suggests a general belief that Soviet policies probably will not change much soon. But some said an opportunity exists for a more conciliatory relationship between the superpowers.

The three living ex-presidents -- Richard M. Nixon, Gerald R. Ford and Jimmy Carter -- had all met Brezhnev. They portrayed him yesterday as a tough yet pragmatic leader who, both Nixon and Carter said, seemed determined to avoid war.

All three presidents negotiated arms-control agreements with Brezhnev, and commentaries by several political figures yesterday focused on that issue as a potential bridge between the two countries.

Carter, appearing on ABC-TV, speculated that in the immediate post-Brezhnev period, the Soviet leadership could become even "more confrontational to establish the strength . . . and vigor of the new leadership." But he also forecast that after this period the Kremlin will probably try to reach an accommodation with the Reagan administration on arms control and other matters.

Carter suggested that the White House "make clear our purpose" and dedication to peace rather than armed conflict and cooperate with Moscow where possible and compete with them when necessary.

Ford, interviewed in Michigan, said that during the transfer of power in Moscow the United States "ought to proceed with putting our act together so that we can responsibly negotiate" with Brezhnev's eventual successor. Ford said that "we should not let our guard down" but "should be prepared to move forward once they have made their choice."

Ford said he hoped Brezhnev's death "will not lead to a wide swing in Soviet policy that would preclude progress toward a reduction in nuclear weapons and the solution of other problems."

Nixon, who had the most contact with Brezhnev, called him "a strong leader of a strong people. As a Russian he was warm, effusive, ebullient. As a Communist, he was a ruthless schemer and a relentless aggressor."

"But he was not a madman. He was a realist," Nixon said in a statement from New York. Brezhnev would take what he could from a weak opponent but compromise with a strong one, Nixon said. "He wanted the world. But he did not want war. If his successor is convinced that we have the strength and the will to resist Soviet aggression, we can avoid both war and defeat without war."

Nixon's and Ford's secretary of state, Henry A. Kissinger, interviewed in Paris, said Moscow would probably be occupied for months and maybe even years in settling their new leadership problems.

Although he expected no fundamental changes in East-West relations, Kissinger said, Moscow "may want to get a breathing space from international advantures" and thus may launch something of a "peace offensive in which they will try to get some immediate tensions out of the way."

The United States, he said, should take advantage of this "by making very concrete proposals on very specific issues."

William Hyland, a former deputy to Kissinger on Soviet affairs, said that "in the near term, I would see no great shift in Soviet foreign policy." But after a period of leadership consolidation, he said, "I think we will see a return to a somewhat more conciliatory policy toward the United States."

Jerry Hough, a Soviet specialist at Duke University, said, "The problem the United States has to face is how to react to any change in Moscow." Professor Marshall Shulman, Carter's former top Soviet adviser, said yesterday that he is not sure that the administration knows itself what it wants in terms of its relationship with the Soviet Union. Hough estimated the chances as 50-50 that the Soviets might use the change in leadership to begin withdrawing from Afghanistan and to reduce pressure in Poland, thus reducing two obstacles to improved U.S. relations.

Another former aide to Kissinger, Helmut Sonnenfeld, also speculated that some Soviet political leaders might like to ease tensions with China and thus reduce the size of their Army kept along that border.

Congressional reaction also reflected a mixture of concern and hope.

Rep. Clement Zablocki (D-Wis.), chairman of the House Foreign Affairs Committee, said he hoped "no undue tensions will arise during the transition" but added that "the advent of new leadership in the Soviet Union does provide an important opportunity for an improvement in U.S.-Soviet relations . . . if the Soviet leadership so chooses."

Sen. Claiborne Pell (D-R.I.), the ranking Democrat on the Senate Foreign Relations Committee, saw the danger of heightened tensions and said, "I would hope that our president . . . would use these days to lower tensions, decrease the resultant dangers and get on a more normal course."

Senate Democratic whip Sen. Alan Cranston of California struck a similar theme, saying, "It is vital that the Reagan administration cool down its bellicose rhetoric . . . and reassure the Soviets" that a fair arms control agreement is being sought.

Sen. Charles McC. Mathias (R-Md.) also forecast "a less stable regime" initially. He called for firm and calm American leadership but one "avoiding any unnecessary provocation." Sen. Paul Tsongas (D-Mass.) forecast "a rough period" ahead. The danger, Tsongas said, is that whereas Brezhnev at least had some control, it may now take two or three years before the Soviets are able to make any moves back toward detente or progress in arms control.

Industrialist Armand Hammer, the 84-year-old head of Occidental Petroleum who has known every Soviet leader since Lenin, said, "My feeling is that if we are patient, there will be a change in Russia. I see change every time I go . . . . Every Russian that gets an automobile is no longer a Communist. They want a better standard of life."

Hammer told CBS-TV in an interview that he expected three men to emerge as an interim collective leadership: Brezhnev's aide Konstantin Chernenko, ex-KGB chief Yuri Andropov and Defense Minister Dmitri Ustinov.