Brazilian Finance Minister Luis Carlos Bresser Pereira, under heavy pressure from commercial bankers and the Reagan administration, yesterday dropped his unorthodox proposal to convert half of Brazil's $68 billion commercial debt into discounted bonds.

The move was seen within the administration as a sign Brazil is ready to start serious negotiations toward resuming payments on its massive debt, which it suspended last February. However, banking sources cautioned that it still will be difficult to reach agreement with the second-largest Third World debtor, which owes public and private lenders $112 billion. They expressed concern that Brazil has not developed a credible plan for restarting debt payments.

Bresser Pereira's announcement came after a morning-long meeting here with Treasury Secretary James A. Baker III, who pronounced the bond plan a "nonstarter." A Treasury spokesman said the meeting produced "general agreement that Brazil's problems should be addressed in a conventional way."

The plan proposed that $35 billion in Brazilian debt be converted into long-term, fixed interest bonds. The principal amount would be equal to the secondary market value of Brazilian debt, which is running at about 55 cents on the dollar. Bresser Pereira told reporters after the meeting that he would propose to banks a "voluntary" system of discounting Brazilian debt through so-called "exit bonds," through which banks trade their debt for bonds. He said he hoped to discuss stretching out some interest payments as well.

The bond plan was dropped, Brazilian Ambassador Marcilio Marques Moreira said in a telephone interview, because "there was a sense that maybe it had been misperceived or misrepresented. So he {Bresser Pereira} rethought the same idea, stressing that it was a voluntary solution."

A senior Treasury official praised the minister's decision to abandon the plan as a sign that "they do wish to remove their arrearages as soon as they can and get fully back into the international financial community." He also quoted Bresser Pereira as saying that Brazil still hoped to negotiate new commercial loans of about $7.2 billion, apparently at interest rates equaling the banks' cost of funds, a plan Brazil announced in July. Bankers have warned that this plan would induce other debtor nations to reopen agreements to seek easier terms.

Commercial bankers were less enthusiastic about the termination of the bond plan, which some speculated had been a trial balloon. Most contended Brazil was a long way from agreeing to a conventional debt-restructuring package of new money in return for economic changes.

"We are probably going to see relatively slow progress on this one," said an executive from a large commercial bank. "I don't think the Brazilians have given up the idea of getting us to agree to something substantially different from the norm." Another banker guessed it would take six months for serious negotiations to get under way.

Bresser Pereira's about-face came after a weekend international debt conference in Vienna at which the bond plan was poorly received by bankers who were present, and just weeks before Brazil is to sit down with its large commercial lenders at the end of September to begin talks on resuming payments. Bresser Pereira has sent his top aides to Britain, other European nations and Japan this month to meet with bankers and government officials.

Other deadlines also may have affected Bresser Pereira's new stance: The international financial world will gather in Washington at the end of this month for the annual meeting of the International Monetary Fund and the World Bank. And a committee of federal regulators, meeting at the end of October, is expected to consider ordering U.S. banks to transfer funds from their loan-loss reserves to a special mandated reserve on the grounds that a portion of the Brazilian debt is uncollectible. Such a move would reduce banks' capital, but would have no impact on earnings unless existing reserves are insufficient.

The Treasury statement said any renegotiation of Brazil's official debt to governments and multilateral lending institutions would require full participation and monitoring by the IMF, but did not impose the same condition on commercial debt talks.

Ambassador Marques Moreira reiterated yesterday that Brazil still opposed following an IMF economic program as a condition for new commercial bank loans. But banking sources said IMF participation was considered vital to any debt restructuring agreement. Without new money from the IMF, they noted, Brazil soon will have to begin repayments of $1 billion a year to that multilateral lender.

"The banks are not keen to put money into Brazil so that the IMF can get repaid," said one banking source.