Yellow Corp., after enduring years of slim margins, is throwing in the towel on its financially ailing Preston Trucking Co.

The proposed sale, set to be made to a management group of three senior Preston executives, seemed to be expected by many in the industry.''They've (Yellow) been having a problem bringing Preston along,'' said Joseph F.H. Cutrona, executive director of the National Small Shipments Traffic Association. ''It looked like they were going to have success with it. The sale doesn't surprise me.''

''This was going to happen,'' said Steve Lewins, an analyst with the New York investment banking firm of Gruntal & Co. ''I think this is cash-positive and I expect a share buyback.''

Mr. Lewins, however, said he will not change his earnings estimate until the deal is done. He had forecast that Yellow would earn between $1.80 to $2.40 a share next year. But with the proposed sale, he revised those estimates upward to $2.70.

''This is great for Yellow,'' he said.

Yellow estimated the financial impact of the sale will result in a second-quarter charge of between $55 million and $65 million.

But the company also said it will realize positive net cash flow mostly from income tax benefits of approximately $20 million.

According to Yellow, the new principals in a stand-alone Preston will include David J. Letke, president; J. Sean Callahan, senior vice president and chief financial officer; and Nicholas J. Marino, senior vice president and chief operating officer. The sale is contingent upon Preston and Yellow officials reaching a contract with the Teamsters. Negotiations on that new pact are slated to begin Nov. 11.

Preston, never strong financially, has often struggled.

Preston lost $6 million on $103.7 million revenue. In the same period a year earlier, the hauler posted an an operating loss of $2.9 million on $103.5 million revenue.

Like many unionized motor carriers, Preston was hurt when shippers increasingly diverted freight as the unionized truckers negotiated feverishly with the Teamsters earlier this year, said A. Maurice Myers, Yellow's chairman, president and chief executive.

The two sides hammered out a new national pact on Feb. 8.

Preston serves the Northeast, Mid-Atlantic and central United States, operating 62 terminals and substations.