Two former secretaries of defense yesterday urged Congress and the Reagan administration to take another look at reviving the never-ratified SALT II strategic arms limitation treaty with Moscow, and expressed doubts about President Reagan's claim that the Soviets had "a definite margin of superiority" over this country.

James R. Schlesinger, who served under Presidents Nixon and Ford, and Harold Brown, who was under President Carter, advanced these views during the second in a series of Senate Foreign Relations Committee hearings aimed at producing a resolution on nuclear arms control policy that can command strong national support.

Sen. Claiborne Pell (D-R.I.) remarked wryly that he was glad to have two witnesses "who presided over that decade of neglect" on military spending that the current defense secretary, Caspar W. Weinberger, says is the cause of so much trouble.

Under questioning by Chairman Sen. Charles H. Percy (R-Ill.), Schlesinger said "we have lost valuable time and, more importantly, the political initiative" both in Europe and among the U.S. population by taking so long in the Reagan administration to get started on new arms talks, which are now called Strategic Arms Reduction Talks, or START.

Schlesinger agreed that there was some validity to administration desires to build up U.S. nuclear forces first.

But he said those goals were overtaken by events and that this country "is losing more . . . both strategically and politically . . . by the failure to negotiate than it would by going to the negotiating table with a somewhat weaker hand."

Percy has also been pushing the White House hard for a U.S.-Soviet summit meeting.

Schlesinger agreed that a meeting would be "desirable," adding that this administration came to office believing that most Americans and allies were not sufficiently aware of the Soviet menace and thus feared that any "palsy-walsy" meeting would detract from their ability to mobilize public opinion.

Although congressional aides say chances are strongly against any revival of the never-ratified 1979 SALT II, a growing number of lawmakers are pushing for it.

One is Sen. John Glenn (D-Ohio), who argued yesterday that SALT II, which is still officially in the Senate although dormant politically, is a "do-able" first step which would require cuts of some 250 missiles in the Soviet arsenal and then lead to the next round of START talks.

Schlesinger said he hoped the administration "would review that possibility of reviving SALT II." Brown, testifying later, "strongly urged this committee to consider again the virtues of the treaty" which he helped design.

The Reagan administration vehemently opposes the Carter-era treaty, claiming that it puts no real limitations on the arms race and confirms Soviet superiority in certain weapons.

Under questioning by Percy, Brown said, "I would have to respectfully disagree" with the assessment of Reagan and Weinberger about Soviet superiority in overall nuclear forces.

Both Brown and Schlesinger agreed that Moscow's land-based missiles were now a threat to knock out U.S. land-based missiles and that the Soviets do have some advantages.

But both officials, as did the chairman of the Joint Chiefs of Staff, Gen. David Jones, the day before, stressed American advantages in bombers and missile-firing submarines, and agreed the United States still has a strong ability to deter any Soviet attack.

"The Soviets do not have, in my judgment, anything like strategic superiority in the sense of a militarily or politically useable advantage in strategic nuclear forces," Brown said.

Furthermore, perceptions of the strategic balance are crucially important because they "affect the political will and morale of governments and publics. Thus, it is important for informed individuals, particularly those with government responsibility, to make every effort to express their judgment of that balance in terms that are accurate--neither alarmist nor complacent," he said.

Schlesinger did not directly respond to the president's claim, but said the United States has significant nuclear strengths that Moscow "may or may not have. I would prefer not to buy a pig in a poke," he said when asked if he would switch forces with Moscow.

Schlesinger said the Soviet space program turned out to be a fraud and stressed that there are many unknowns about Soviet weapons.

Schlesinger, a former CIA director, stressed there were great unknowns about missile accuracy and technical failure on both sides. "Given the spotty Soviet history in dealing with modern technologies, one would hypothesize that this must be a constant worry of the Soviet leaders.

"We ourselves know a great deal more about helicopter operations and maintenance than we do about actual missile operations. Yet if we recall the abortive rescue operation in Iran in 1980, even we, with a far more impressive history of technical success, should bear in mind this salient element," he said.