The Justice Department yesterday sued the Buffalo school board, the Buffalo teachers' union and two other school employe unions on charges of illegally denying sick leave and disability benefits to pregnant employes.

The suit, filed in U.S. District Court in Buffalo, seeks an injunction to bar the defendants from continuing to regard pregnancy, childbirth and related complications differently from other medical disabilities. The suit also asks financial compensation for an undetermined number of female employes who have been denied such benefits in the past.

The Justice Department has authorized two other discrimination suits under the 1978 Pregnancy Disability Act involving denial of pregnancy benefits and is investigating several additional cases for possible legal action, according to knowledgeable sources.

The defendants in yesterday's suit could not be reached for comment. They include the Buffalo Board of Education, Buffalo Teachers Federation, Local 650 of the American Federation of State, County and Municipal Employes and a second union representing professional, clerical and technical employes.

The government charged that the Buffalo school system violated the law by signing collective bargaining agreements that discriminate on the basis of sex because they do not define pregnancy and childbirth as a medical disability.

As a result, the suit charged, female employes who have exhausted their sick leave are denied additional sick leave days for pregnancy and childbirth, although such extra leave is available to other disabled employes.

The school board also excludes pregnancy-related complications from coverage under its long-term disability insurance policies, the suit contends. Women on maternity leave must file monthly statements and submit doctors' notes explaining why they cannot return to work, a requirement that does not extend to other disabled employes, according to the lawsuit.

The defendants also discriminate against men by denying them child-care leave on the same basis as women, the government said. In addition, health insurance must be placed in the husband's name if both spouses are school employes.

The Justice investigation began when several Buffalo school employes complained to the Equal Employment Opportunity Commission about the restrictions on pregnancy benefits. After failing to negotiate a voluntary settlement, the commission, which does not have the authority to sue local and state governments, referred the case to Justice.

In a similar case now before the Supreme Court, the commission contended that a private firm illegally denied pregnancy benefits to the spouses of male employes. The commission was upheld in lower court rulings.