The parole request of former Maryland governor Marvin Mandel was turned down again yesterday.

The nine-member U.S. Parole Commission, after a hearing on Thursday, reaffirmed its earlier decision that Mandel, convicted of political corruption, must remain in federal prison until May 4, 1982.

The commission also reaffirmed a Jan. 13, 1982, parole date for Mandel's codefendant W. Dale Hess, and shortened to that same date the sentence of another codefendant, Harry W. Rodgers III. Rodgers previously had been assigned the same release date as the ex-governor.

In an unrelated action, the commission moved up the release date of former U.S. representative Charles C. Diggs from Sept. 9, 1981, to May 25, 1981. Diggs, who had been chairman of the House District Committee, began serving a three-year sentence July 25 at the federal prison camp at Maxwell, Ala. He was convicted in October 1978 of inflating the salaries of staff members and then forcing them to kickback the extra money, totaling more than $40,000 to him. As always, the commission announced no reasons for its decisions.

Yesterday's action exhausted Mandel's appeal process within the penal system, although his lawyers could bring a court action attempting to overturn the parole board's decision.

Mandel began serving a three-year sentence at the Eglin (Fla.) federal prison camp May 19. His original four-year sentence was reduced to three years by the trial judge, after Mandel and the others lost a two-year battle to overturn their convictions on charges of mail fraud and racketeering. Despite being sentenced to three years in prison, Mandel will serve only two, with one year being taken off for so-called "good time." That reduction has nothing to do with the parole procedures.

Mandel and five codefendants were convicted in August 1977 by federal-court jury in Baltimore. Federal prosecutors argued that while he was governor, Mandel accepted gifts worth more than $300,000 from his codefendant friends in exchange for his help with legislation benefiting them in connection with their secret purchase of the run-down Marlboro Race Track in Prince George's County.