The American Postal Workers Union agreed yesterday to delay mailing out contract ratification ballots for 48 hours so a federal judge can rule in a challenge by union dissidents to the legality of the referendum.

The Pittsburg Metro Area Postal Workers Union (APWU) contends that the national bargaining advisory committee preempted the vote by rejecting the contract on Thursday, even thought the committee later specifically called for submission of the pact to the union's nearly 300,000 members across the country.

At a hearing before U.S. District Court Judge Barrington D. Parker, APWU President Emmet Andrews agreed to delay mailing the ballots from Wednesday to Friday of next week. A hearing on the Pittsburg group's request for an injuction will be held Tuesday and Parker said he would rule by Friday.

Andrew said later he "deplored" the suit as an infringement on the "democratic rights" of union members to vote on a proposed contract.

Three other postal unions are preparing to submit the contract, which was negotiated July 21 with the Postal Service, to their members for ratification next week. Only the APWU, the largerst of the four, faces a legal challenge to the ratification process, which is expected to take about two weeks. The APWU's New York members are also scheduled to takea strike vote Monday night.