House Democratic leaders, meeting with liberals rebelling against the compromise defense authorization bill, agreed yesterday to hold a separate vote on whether the measure should be reduced by $10 billion.

In another development clouding the future of the Pentagon's fiscal 1986 budget, House Appropriations Committee sources said that there is not enough time to finish the budget before the fiscal year begins Oct. 1 and that a continuing resolution keeping funding at the fiscal 1985 level is virtually certain.

Rep. Charles E. Schumer (D-N.Y.), a member of the Budget Committee, said he and others at yesterday's meeting with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) argued that they did not want to be put in the position of "looking soft on defense."

Rather than having the usual up-or-down vote on the conference report as a whole, Schumer said, the liberals want to project an image of insisting on "a strong but lean" national defense.

Their amendment would reduce the compromise bill from the $302.5 billion voted by the Senate to the $292 billion approved by the House when the fiscal 1986 authorization bill made its first lap on the legislative track.

In a separate vote on dollar totals, there is "no question" that the bill will be reduced, O'Neill said.

Schumer predicted that "we'll get the votes of the vast majority of Democrats and a good many Republicans because of the perception that the Pentagon is wasting money by buying such things as $700 coffeepots."

House Democrats are scheduled to caucus Wednesday on what to do about the conference report, which has passed the Senate. Votes on whether to send it back to conference could come as early as Thursday, according to lawmakers in the negotiations yesterday.

Although the amendment to lower the total by $10 billion appears to be attracting the most support, Schumer and others said another amendment is in the works to send the procurement bill back to conference with a reference to delete the authorization to resume nerve-gas production.

House Armed Services Committee Chairman Les Aspin (D-Wis.) was at the leadership meeting with Budget Committee Chairman William H. Gray III (D-Pa.).

According to participants, Aspin remarked that, in this first year as chairman, he has learned to hammer out an acceptable bill in committee but that he must do better in House-Senate conferences on rival measures.

"I told them, while we're away, [Defense Secretary Caspar W.] Weinberger kills two programs and all we did was add money to our budget," Schumer said in relaying his remarks in the leadership meeting.

He said he was referring to Weinberger's cancellation last week of the Army's Divad antiaircraft gun and the Air Force offer to cancel procurement of the T46 training plane to lower its fiscal 1987 budget.

Further clouding the future of the Pentagon's fiscal 1986 budget is the illness of Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House Appropriations Committee's defense subcommittee. In a separate legislative process, that panel recommends how much money should be appropriated.