In a new effort to end Washington's teachers' strike, Mayor Marion Barry asked a Superior Court judge yesterday to reinstate until Sept. 15 the teachers' old union contract with the school board while negotiations continue on a new contract.

Barry's court request, which would grant one of the teachers' key interim demands, served to head off a threatened work stoppage this morning by two union locals representing about 5,000 custodians, cafeteria works, secretaries and other nonteaching school system employes.

Late last night, members of both locals approved by voice vote a decision to delay any strike action until the court can rule on Barry's new effort to end the walkout.

School Superintendent Vincent Reed had said earlier yesterday that the schools would remain open regardless of any moves by the non-teaching employes to join the strike, which began March 6.

In interviews yesterday, many cafeteria workers and custodians told reporters they had serious doubts about joining the strike.

"I don't think it's necessary for us to get involved in this," said one cafeteria worker at Eastern Senior High School. "They [the teachers] have different jobs form us. I don't want to get involved in somebody else's mess."

barry's court motion which will be heard at 8:30 a.m. today by Judge Gladys Kessler, draw a warm response from striking teachers and sharp oppostion from the school board.

"It's a very good move," said Harold Fisher, assistant to Washington Teachers' Union President William Simons. "This is basically what we've asked for before. . . If the board had extended the contract like this in February, we would never have been out."

But board member R. Calvin Lockridge, who voted against extending the contract on the grounds that it was too pro-union, said "The mayor says he's intervening as a 'friend of the court.' He's really intervening as a friend of the union. He's trying to save the union from paying those fines [for violating Judge Kessler's earlier no-strike orders], and trying to reinstate a contract like the one he signed in 1972 when he was [school] board president."

I think Barry is guilty of weakness and fear," said school board President Minnie S. Woodson. "I think the union gathers strength from it. They figure, 'We don't have to negotiate with the board because the mayor is with us.'"

Negotiators for the board and teachers' union met for five hours yesterday with no progress reported.They will meet again at 10 a.m. today.

"They're not doing what you would call meaningful negotiations," said a spokesman for the Federal Mediation and Conciliation Service, which is providing mediators for the talks. "They seem to be waiting for a decision in court."

Last Saturday Kessler issued an injunction declaring the teachers' walkout illegal and barring strike activity by all school system employes, not just teachers.

The order also reinstated the old teachers' union contract for one week and barred any "advance action" against teachers while negotiations continue. But at a mass meeting Sunday the teachers voted to defy the order because the contract extension was no long enough.

School officials said 52 percent of the teachers and about 55 percent of the students were absent from school yesterday. The figures are about the same as those reported for the last week by the board, but considerably below estimates made by the union.

In his motion yesterday, Barry said he wanted to enter the court case because the strike is having an "immense effect upon. . . the education, health, safety and good order of the City politic."

An aide to the mayor said, "You've got to try to settle this conflict somehow or the mayor is going to have to deal with all of those children in the street and all of those union people. That's something the school board doesn't have to deal with. . . The people on the school board just sit in a tent."

The aide said other unions flet the board was trying to "bust" the teachers' union.

However, several custodians interviewed yesterday recalled that teachers continued to work in 1976 while their union staged a sick-out to protest undermanning.

"I feel we should not go out with them because when we went out they brought brooms and mops to clean up the schools. They laughed at us," said Harry Shaw, a custodian at Eliot Junior High School in Northeast Washington.

Reed said yesterday the school system was making plans for Saturday classes, after-school sessions and tutoring in an effort to make up work lost because of the strike.

He also said there would be summer school for remedial and advanced work for the first time in three years. But he said he was not making plans to extend the regular school year in June , as the striking teachers' union has demanded.

"A longer school year would not necessarily mean better quality," Reed said. "We feel it's better to reinforce the regular classroom."

Reed added that if the strike continues for another week, "we are goint to take serious look into terminating some teachers. . .

"There are plenty of people who want to teach in Washington," Reed continued. "If the people out there now don't want to work, then we will have to move to get some people who do. Lord knows, I don't want to do that. But as sad as it makes me, if this thing continues, I'll have to move to do that."

At today's hearing, Judge Kessler also will consider a school board motion to begin collecting the fines she has imposed against the teachers' union and its officers for violating her contempt order of March 12.According to school board lawyers, the fines now exceed $1 million, but union lawyers estimate the total at about half that.

Also contributing to this story were Washington Post staff writers LaBarbara Bowman, Milton Coleman, Jack Eisen, Jean S. Fugett Jr. aand Joseph D. Whitaker