In its first move to crack down on news leaks, the Pentagon began dismissal proceedings against one of its officials on charges of disclosing classified information to unauthorized persons, including a former Pentagon official. John C. F. Tillson, director of manpower management until his temporary reassignment two weeks ago to the Office of Economic Adjustment, was one of about 25 people who attended a top secret briefing of the Defense Resources Board Jan. 7 at which it was disclosed the Reagan administration's arms buildup might cost $750 billion more than the $1.6 trillion earmarked for it over the next five years.
Details of the briefing were leaked to the Washington Post, which published the story Jan. 8. Deputy Defense Secretary Frank Carlucci ordered those at the session to take lie detector tests. Authorities said Mr. Tillson failed the test , according to Mr. Tillson's attorney.
George Wilson, the Post reporter who leaked the story, wrote to Defense Secretary Caspar Weinberger saying that Mr. Tillson was not his source and that he was prepared to ''take an oath on it,'' Mr. Tillson's lawyer said. The lawyer said his client, a West Point graduate and Army company commander who received two Silver Stars for valor during the Vietnam war, denied the allegations and planned to appeal to the Merit Systems Protection Board if he is fired.