Senate Majority Leader Robert C. Byrd (D-W.Va.) stepped up pressure yesterday for a vote Tuesday to end the month-long filibuster against the labor law revision bill, warning that further concessions before the vote are unlikely.

Despite the Senate's five votes against choking off debate over the past two weeks and signs of uneasiness among antifilibuster forces, Byrd said he still believes the Senate will produce the 60 votes necessary to end the filibuster - two more votes than it had last week.

But he refused to say how long he would keep the bill on the floor if the filibuster continues, although he noted pointedly that he is not worried about the 1,200 amendments that face the bill after cloture of the backlog of other legislation facing the Senate before its campaign recess.

Byrd, speaking at his regular Saturday news conference, sharpened his criticism of the filibuster leaders, accusing them of trying to kill the bill rather than modifying it to meet "legitimate" objections.

"Obviously the opposition doesn't want any bill," he said. "They're not willing to vote on any amendments. They want to kill the bill."

Noting that he has offered modifying amendments, Byrd said he doesn't "know what room there may be for compromise," and then said flatly, "We're past the point of responding to legitimate concerns."

However, as a further inducement to invoke cloture on Tuesday, he held out the likelihood of "substantive" amendments after the filibuster is broken.

Tuesday's vote thus appears to be crucial for the labor bill, a measure backed by the Carter administration and organized labor to make it more difficult for employers to counter union organizing and bargaining efforts illegally. It would set deadlines for union representation elections and stiffen penalties for violations of labor laws.

The bill's proponents have viewed last week's cloture drive as probably decisive, but feel short in two successive tries, obtaining only 58 votes. Tuesday's sixth cloture vote would tie the Senate's record for filibuster-ending attempts, established in 1975 during debate over a disputed New Hampshire election.

Responding to expressed fears of more delay from a post-cloture "filibuster of amendments," Byrd said the amendments could be wrapped up in a week to 10 days, suggesting obliquely that "new precedents" may be set for dealing with that kind of delay.

"The number of amendments don't scare me . . . I'm not easily stampeded by amendments," he said. But he wryly conceded that his image as a legislative tactician "who makes the trains run" may now have been "thoroughly exploded."