President Reagan told two bipartisan groups of congressmen yesterday that the United States will demonstrate new flexibility when the Strategic Arms Reduction Talks (START) resume in Geneva this week in an effort to win a breakthrough agreement with the Soviet Union.

"We'll be as flexible as necessary," Reagan later told reporters at a White House ceremony.

Administration officials and congressmen who discussed the two meetings said that Reagan will extend the life of the President's Commission on Strategic Forces and that retired Air Force Lt. Gen. Brent Scowcroft, a popular figure on Capitol Hill, will continue as its chairman. The president also pledged in general terms to adopt the Scowcroft commission's recommendations on arms control, moving toward a count of warheads rather than missile launchers in the Geneva negotiations.

The president's willingness to revise the U.S. negotiating position, which he will announce officially today, appeared to reassure most of the congressmen who met with him yesterday.

"Everybody there was impressed with the sincerity of the president," said Rep. Norman D. Dicks (D-Wash.) after a meeting between Reagan and House members. "On the big issue of throw weight, he seemed to be be willing to take a very flexible approach."

Throw weight is the lifting power of missiles, in which the Soviets are considered to have a 3-to-1 edge. Reagan reassured the House members, who have urged him not to make throw weight a major consideration in the negotiations, that he would not insist on a rigid formula.

Rep. Albert Gore Jr. (D-Tenn.), one of the congressmen most concerned that the throw-weight issue might prove a stumbling block, said after the meeting that "the president is leaning against an unrealistic demand for equal throw weight" but that Defense Secretary Caspar W. Weinberger had "argued strenuously" about the importance of throw weight. Gore said he emerged from the meeting with the impression that Reagan was leaning against Weinberger's position.

In a subsequent meeting with senators, the dominant issue was the question of "build-down," under which both sides would scrap two older missiles for each new one they deploy.

Robert C. (Bud) McFarlane, deputy assistant for national security affairs and principal briefing officer for both meetings, assured the senators that the administration was committed to trying to incorporate build-down in its negotiating position. However, he listed seven problems that have to be solved, including the key issue of verifying that the older missiles had been replaced.

Sen. William S. Cohen (R-Maine), one of the originators of the build-down idea, acknowledged the complexities but said the administration "ought to approach it in the spirit of making it work rather than emphasizing the pitfalls." After the meeting, however, Cohen said he was "satisfied that the administration is now focusing on build-down in a positive way."

The administration officials who briefed the congressmen said that build-down will not be a part of the revised position that Reagan will announce this week. Reagan also said he will not announce numbers in the U.S. proposals, telling the congressmen that he did not believe that "negotiations could be carried out in the headlines," according to participants in the meetings.

Rep. Les Aspin (D-Wis.), one of the Democratic congressmen whose support of the MX the administration considers most significant, said afterward that he expected the United States to revise its position to propose a limit of 5,000 nuclear warheads and 1,100 to 1,200 missile launchers. The Soviets are proposing a limit of 1,800 launchers.

Originally, the U.S. position at START was for a limit of 5,000 warheads and 850 launchers.

The shift to a larger number of launchers would be consistent with the recommendations of the Scowcroft commission for an eventual conversion of both sides to smaller, single-warhead missiles that are considered less destabilizing than such multi-warhead missiles as the MX.

Aspin criticized the president with faint praise yesterday, saying, "Reagan is doing the minimum he has to do but that's all we expected him to do."

He also said that the administration "was not trying very hard" to put in build-down because of opposition to the concept. This was sharply disputed by administration officials.

Overall, however, the congressmen emerged from the White House with compliments for Reagan and indications that they remain supportive of the Scowcroft commission's recommendations for deploying 100 MX missiles in existing Minuteman silos.

McFarlane said in his briefing of House members that the president would like to extend the life of the Scowcroft commission "to the turn of the century" but that some of the members were reluctant to serve indefinitely. Afterward, administration officials said the commission's life would be extended to January, 1984, and that ways would be explored to name a permanent arms control advisory body after that.

But Thomas C. Reed, the commission vice chairman, will not continue in that post as earlier reported in The Washington Post, officials said.

One source said that national security officials had concluded that Reed's involvement in a 1981 stock deal, the subject of investigation by a federal grand jury and a congressional committee, was "too hot to handle politically." Other White House officials concurred, they said.

The MX suffered a minor setback yesterday when the Democratic-controlled House by voice vote approved an amendment to defer $20 million in MX construction facilities until Congress authorizes production of the missile. Republicans offered no opposition.

Meanwhile, MX opponents were trying to put pressure on moderate Democrats who supported the missile in initial votes.

Dicks said that a group called the Peace Development Fund had purchased radio advertisements in the districts of several moderate Democrats saying that support of the missile was "a terrible mistake." Dicks said that he thought the ads were "counterproductive" and added that he would continue to support the MX.