Defense Secretary Harold Brown urged Congress yesterday to hold off on legislation banning military unions until Pentagon regulations to deal with them are tried.

Declaring that rules containing unionization efforts within the military have a better chance than a low of surviving constitutional challenges, Brown recommended trying them for "nine or 12 months."

"I do not rule out the possibility that legislation may be needed in the future." Brown wrote the Senate Armed Services Committee yesterday. But first the rules - to be issued within 30 days - should be given a trial, he said.

Sen. Strom Thurmond (R-S.C.), sponsor of a bill to outlaw military unions, attacked Brown's position as "weak-kneed" in the face of the unionization threat.

Thurmond voiced his attack during a committee hearing yesterday where representatives of each of the armed services expressed fears about military unions.

"Disastrous" was the term used by Gen. Bernard W. Rogers, Army chief of staff, who testified as the represenatative of the Joint Chiefs of Staff. Rogers, in a departure from Brown's position, said that legislation would be best way to deal with the threat of unionization.

The rules Brown wants to try first, Thurmond said, would an "an after the fact remedey" that disregards progress already made in efforts to unionize the military.

"Let's nip this thing in the bud," Thurmond said, by outlawing military unions outright rather than giving base commanders the responsibility of dealing with them.

The Pentagon rules for dealing with unions, Brown wrote the committee, will not forbid joining them unless they engage in such "prohibited conduct" as strikes, slowdowns or coercive picketing.

Even if a union has engaged in one of those activities, Brown said, joining it would not be illegal unless the base commander had determined that "the organization poses a clear danger to discipline, obedience to lawful orders or chain of command of the armed forces."

"The most complex" part of the draft directive, Brown continued in his letter, concerns rules for a service person joining a union. The other draft rules are more direct, including prohibitions against strikes, slowdowns or coercive picketing.

The directive will also forbid what Brown called "collective job action: activity by two or more persons that interferes with or obstructs a duty assignment," such as orders to capture a hill.

The Defense Department "can meet the potential unionization problem" with the "comprehensive directive" now being drafted, Brown said. "I hope that that the committee will hold legislation in abeyance."

The American Federation of Government Employees (AFL-CIO) currently is polling to see if its members want to try to unionize the military. The results are expected to be announced in October.