One major surprise and many vital clues about the nature of power within the Kremlin Politburo during the waning Brezhnev era emerged from the Soviet-American summit.

The biggest surprise for Kremlin watchers seeking information on the vital question of a successor to Leonid Brezhnev was the insignificant role played at the meetings by Konstantin Chernenko, 67, the Brezhnev confidant whose elevation to full Politburo membership last year marked him to many Western annalysts as a possible successor.

At the same time, Defense Minister Dmitri Ustinov, 70, showed an easy informality with Brezhnev, intervening in the discussions on his own without asking the leader's permission, in a way that suggested near equality between the two men.

By contrast, Foreign Minister Andrei Gromyko carefully sought Brezhnev's acquiesence before he offered his own views or rejoinders, according to one member of the official American delegation who atttended the plenary sessions of the summit conference.

Gromyko followed this practice even though he took the major responsibility for explaining many Soviet policies, this source said. Gromyko's long expertise and continuing vitality are the obvious reasons for the major role he played in the formal talks, the U.S. source said.

Brezhnev did not engage in any spontaneous substantive discussions with President Carter aside from his well-publicized personal guarantee that the Soviets will not build more than 30 Backfire supersonic bombers a year.

The Soviet president and Communist Party chairman read from prepared position papers, according to this source, who said that as the summit progressed, Brezhnev's vitality flagged until we could hardly be understood by his own veteran translator, Viktor Sukhadrey.

Several times during a late formal session, Brezhnev appeared to some Americans to have dozed off. The other Soviets never moved to rouse him and after a while, Brezhnev opened his eyws and resumed an active role.

Ustinov, civilian head of the Soviet strategic weapons industry, was made defense minister by Brezhnev in 1976. He was described as acting "almost as a peer" to Brezhev during the formal sessions. Without checking with Brezhnev, he intervened in the president-to-president talks on his own, several times apparently questioning or challenging the Americans on their positions.

The special personal nature of the Brezhnev-Ustinov relationship seened to one U.S. source to have been emphasized in one offhand incident. Brezhnev suddenly and unceremoniously dug an elbow into Ustinov's ribs during one of the sessions and directed him to "open that bottle" of mineral water on the conference table. Ustinov complied promptly.

Ustinov's deputy, Marshall Nikolai Ogarkov, who is chief of the Soviet General Staff is said to have played no active role in the plenary sessions, but to have paid close attention to the dialogue. The U.S. source said Ogarkov made no attempt to hide his interest, pushing forward in his chair to catch Brezhnev's words and the American positions.

The source said Ogarkov and U.S. Defense Secretary Harold Brown seemed to have broken through the formalities of the talks and to have established a personal rapport.

Ogarkov clearly intrigued the Americans by his attempts to speak English with some of them and by what perceived to be a genuinely broad interest in all the subjects raised at the summit.

The Americans seemed reassured that Ogarkov will be a major figure of continuity at a time of impending change for the Kremlin - and, by extension, for Washington as well.

While no American source would suggest that end of the Brezhnev era is imminent, the signs of continuity in the Soviet delegation nevertheless seemed reassuring to them.