A federal judge is expected to rule Wednesday on whether Crowley Maritime Corp. must post a large cash bond in order to appeal a recent antitrust judgment against a Crowley subsidiary, sources said Monday.

Crowley asked Judge Henry A. Mentz Jr., U.S. District Court in New Orleans, last week for relief from the $50.4 million bond required under Louisiana law. Unless bond is suspended, Crowley has until April 10 to come up with the money.''We're preparing and will soon file post trial motions to set the jury verdict aside," said Dando B. Cellini, lead counsel for Trailer Marine Transport Corp., the Crowley subsidiary. "If not granted, we will prosecute an appeal. We are confident the verdict will be overturned."

A New Orleans jury found in February that Trailer Marine had a hand in forcing American Caribe Lines Inc. out of the mainland-Puerto Rico trade in 1984. On March 2, the jury awarded $14 million in damages to Zapata Gulf Marine Corp., a creditor of American Caribe. The damage award is automatically tripled under U.S. antitrust law.

Zapata, which chartered barges to American Caribe, is entitled to any claims or damages incurred by American Caribe under the latter's Chapter 7 asset-liquidation agreement with its creditors.

Crowley officials said they would appeal the judgment. But in order to do so under Louisiana law, the company must post a bond equal to 120 percent of the damage award.

Joseph Mole, an attorney with the New Orleans law firm of Lemle & Kelleher, which represents Zapata, said the judge who heard the case has the discretion to suspend or reduce the appeals bond or to come up with some alternative to a cash bond.

The current legal maneuvering stems from a suit filed by Zapata against the three largest ship lines in the mainland-Puerto Rico trade, Trailer Marine, Navieras de Puerto Rico and Sea-Land Service Inc.

The suit charged that the three carriers boosted their freight rates by 20 percent in 1983 and early 1984, shortly before American Caribe entered the trade offering lower rates. The new carrier's arrival prompted the big three to undercut American Caribe's rates, but only to those ports American Caribe called, according to the suit.

Unable to operate profitably, American Caribe filed for Chapter 7 bankruptcy on Nov. 28, 1984.

Trailer Marine was the only one of the three lines to fight the suit in court.