Senate Majority Leader Robert C. Byrd (D-W.Va.) yesterday assigned number one priority in next year's Senate session to SALT II and said its chances are looking up.

Byrd met with reporters to look backward and forward as House and Senate ended their first session of the 96th Congress and left town for a month.

Byrd, who has put his reputation on the line for SALT as on other foreign policy issues, said he believes the decline in the arms limitation treaty's fortunes has "botomed out. I think momentum is building for SALT." Byrd said factors working in behalf of the two-third vote the treaty needs include action by the NATO allies to modernize forces, the administration's plan for a five-year defense buildup and the "firm, reasoned stand" by President Carter in the Iranian crisis.

Congress returns for its second session Jan. 22, unless reconvened by leaders earlier to deal with an emergency. Byrd said he hopes to begin the Salt debate in January or early February and bring it to a vote after four or five weeks, "Everything has been said that can be said," he observed.

"The debate will be interrupted to vote on conference reports on the energy and oil tax bills as they are ready," he said. c

The SALT debate will not be delayed until the necessary votes are in hand, Byrd said. "I daresay we won't know until the roll is called where every vote is." Byrd would not comment on whether the Salt debate schedule would be altered if Iran still holds American hostages when Congress returns.

Looking back at the first session, Byrd said he was disappointed not to have had SALT approved and the energy bills enacted but said in retrospect that was impossible. In general, he said, Congress "responded in a responsible way to the mood of the American people" with caution, few new starts and attempts to balance the budget. He predicted the 96th will end its second session with a good record.

Congress was kept up late the final nght of the session waiting for final agreement on the Chrysler aid bill which it felt had to be passed before the end of the year.The house passed the conference report and quit at 11:33 p.m. but the Senate talked on about Chrysler and a controversial judicial nomination until 3:17 a.m.

House and Senate will meet periodically in pro forma session but transact no business until Jan. 22. They recessed rather than adjourned to a specific date so leaders could call them back into session quickly. Had Congress adjourned for a month it could be called back before then only by the president.