RIO DE JANEIRO — WHEN Marcelo Paiva's Army sergeant asked why he had left a blank space on his induction papers as to whether or not his father was dead or alive, the teenager retorted: ''Go ask your superiors, they know better than I.'' It was 1977. The sergeant did check and discovered what Mr. Paiva had long suspected: his father, the former socialist federal Congressman Rubens Paiva, had been ''disappeared'' by Brazil's military soon after his arrest six years earlier. Marcelo Paiva, now an author and newspaper reporter, has been a leading critic of the four democratic governments, which since 1985 have failed to provide answers for more than 100 such disappearances during the military dictatorship of 1964 to '85. That may finally change, however, with a law expected to be introduced this week that would acknowledge the military's responsibility for the deaths, issue official death certificates, and compensate each family with $108,000 to $165,000. Mr. Paiva is encouraged by the move, an initiative of President Fernando Henrique Cardoso, who took office last year and is himself a former designated ''subversive'' and former poker buddy of Paiva's father. ''It's an important step that should be approved as quickly as possible by the Congress,'' Paiva said in an interview at Stanford University in Palo Alto, Calif., where he is a Knight fellow. Yet other relatives of the disappeared remain dissatisfied. For one thing, they want the government to take action, not only in the 136 cases of Brazilians it officially lists as disappeared, but for an additional 217 victims long known to have died in military custody, but registered as suicides or fatalities of accidents or shootouts - explanations many relatives do not believe. Most important, however, human rights activists and victims' families want the military to open up its archives so they can begin a case-by-case investigation of how victims died, who was responsible for their torture and deaths, and where they are buried. ''Our relatives were murdered by the dictatorship, but were said to be bandits who died in shootouts,'' said Jaime Wright, a Presbyterian minister in Sao Paolo, whose brother Paulo disappeared in 1973. ''It's an obligation that the state has with history.'' Some relatives of the disappeared charge that President Cardoso has promised military leaders he will not force them to provide such information. Press reports say the president is worried about a possible ''Chilean crisis,'' referring to the recent showdown between the democratic government of President Eduardo Frei Ruiz-Tagle and the Chilean military over convicted Gen. Manuel Contreras. The general has refused to serve a seven-year prison sentence for the murder of military critic Orlando Letelier in 1976 in Washington. So far, the military has supported President Cardoso's bill, but has made known the bill's limitations. Just last week, Navy Minister Mauro Cesar Pereira reminded reporters that a 1979 amnesty law prevents prosecution of military agents for acts of violence during the dictatorship, warning that attempts to investigate such cases would only open old wounds. ''We are not going to look for events from the past that do not build anything,'' he said.