An administrative law judge has dismissed a New York labor union's attempts to justify its firing of former Secretary-Treasurer Mario Montuoro as a transparent "sham."

The ruling paves the way for Montuoro, who has accused Labor Secretary Raymond J. Donovan of being present at a $2,000 payoff involving Montuoro's union, to claim back pay and other benefits totaling more than $50,000.

Julius Cohn, an administrative law judge for the National Labor Relations Board (NLRB), concluded a one-day hearing here Friday by announcing that he would enter an order requiring the union, Laborers Local 29, to reinstate Montuoro and pay him the income he has lost since May of 1978.

Montuoro was dismissed on May 12, 1978, by Local 29 President Louis Sanzo after an unsuccessful effort by Montuoro to promote the election of a black, Charles Smith, to replace Sanzo.

According to testimony in an earlier proceeding, Sanzo told a Local 29 bookkeeper that the "real boss" of Local 29, reputed Mafia soldier Samuel (Big Sam) Cavalieri, had instructed Sanzo not to let "the niggers" take over.

Another NLRB judge held in 1979 that Montuoro had been fired from his full-time job as assistant administrator of the local's pension and welfare fund, as well as from his post as secretary-treasurer, for his work on behalf of black union members and for other activities "protected" by federal labor law.

The 1979 decision also summed up the testimony of Montuoro and his colleagues as "clearly and forthrightly given" for the most part. Local 29's witnesses were described as "cryptic, guarded, evasive and unconvincing."

The five-member NLRB upheld the decision but the union did not reinstate Montuoro until last May, in the face of an enforcement order issued by the U.S. Court of Appeals here. Then, according to accounts entered into the record Friday, the trustees of Local 29's pension and welfare funds rehired Montuoro for only a few minutes before firing him again, purportedly because of a fraudulent, five-year-old dental claim.

Local 29 pension fund attorney Robert Brady contended that facts underlying the new discharge had only recently been discovered, but Montuoro and his attorney, Arthur Schwartz, insisted that the details had been well known to union officials for years.

Judge Cohn held that the union had had ample time to raise the issue and could at least have submitted it to the NLRB or to the courts before last year's enforcement decree.

Local 29 lawyer Brady said he intended to appeal the rulings to the NLRB. But he said he would not contest the estimates of NLRB lawyers that Montuoro had $50,770, not counting interest, owed to him as of Dec. 31. If Cohn's ruling is upheld, Montuoro will keep earning back pay until the case is settled.