Housing and Urban Development Secretary Patricia Roberts Harris said yesterday her department will work harder to develop lawsuits against large property owners or agents who discriminate in housing sales or rentals.

Harris cited a national survey conducted for HUD by the National Committee Against Discrimination in Housing and said it "confirmed the appalling facat that black people still encounter unconscionable racial discrimination."

The study was released yesterday at a fair housing conference here sponsored by HUD and the committee. The report said the blacaks, in a typical search for housing in which they go through four rental agents or brokers, have a 75 percent chance of encountering discrimination in the rental market and 62 percent in the sales market.

Harris, addressing some 500 delegates attending the conference, acknowledged that 10 years after passage of the national Fair Housing Act, which bans discrimination in the sale or rental of most housing, "we have seen little diminution of inequality."

She also conceded afterward that HUD has not been "operating as effectively as we could have" in the areas of conciliation and in referring cases that cannot be conciliated to the Justice Department for possible legal action.

Under the 1968 law HUD has no enforcement power, but it can try to resolve problems between retal agents or brokers and people who claim to be discriminated against. The agency receives about 3,000 complaints a year. In 1976, the last year for which statistics are available, it tried to conciliate in 1,014 cases and succeeded in 602. Its backlog of cases open more than 90 days was more than 800 last October, but Harris said yesterday that as of last week "we reduced it to 221".

Frank Schwelb, head of the housing and credit section of the Justice Department's civil rights division, said that over the years HUD has referred cases against about 100 defendants and that Justice had brought suit in only about a quarter of those cases. Most of the 280 lawsuits it has filed against some 700 defendants "have been developed by our ownlawyers," Schwelb said.

In many cases HUD's evidence was too old or did not support a pattern of discrimination, which the 1968 law requires Justice to prove, he added.

Harris said yesterday HUD plans to "create teams to review situations where multiple complaints have been filed" and to focus "on the large property owener or agent where we can have a significant impact.

"We believe in example and intend to create a few," she said.

She announced the agency will fund a $500,000 demonstration project using local civil rights or fair housing agencies to take complaints, monitor real estate practices, and develop information for possible lawsuits. She said the National Committee Against Discrimination in Housing will received a $28,000 contract to develop guidebooks to help local groups combat housing bias.

Harris also called for passage of bill introduced by Reps. Don Edwards (D-Calif.) and Robert F. Drinan (D-Mass.) which would enable HUD to conduct hearings on bias complaints and to order anyone judge guilty of discriminating to stop doing so and to sell or lease to the complainant and/or pay a penalty.