SAO PAULO--Brazil's real on Wednesday traded at the weakest level against the greenback in more than six months, but failed to break past BRL2.1 per dollar as traders remained wary of central bank intervention.

After exiting active trading on Tuesday at BRL2.0797, the real weakened to as much as BRL2.0971 in Wednesday trading on concern about Europe after euro-zone minister failed to reach a deal on an aid package for Greece.

The breakdown in talks means Greece has to wait longer to get a long-withheld chunk of aid of at least EUR31.5 billion, with Eurogroup president Jean-Claude Juncker saying on his way out of the meeting he doesn't know when Greece will get the next installment.

In Brazil, traders almost pushed the real past BRL2.1 per dollar, bolstered by comments in a Valor Economico interview of President Dilma Rousseff Tuesday, in which the president said she didn't want an "overvalued" Brazil currency.

The real has been trading between BRL2 and BRL2.1 per dollar for the past six months, due in large part to the central bank holding auctions whenever the real approached those limits. But with the central bank quiet in recent weeks, some traders saw the chance to break past the BRL2.1 mark and tested the bank's willingness to intervene.

However, a central bank statement shortly before 11:00 a.m. local time, saying that the bank would change the way it intervenes in the currency market, stopped those traders short.

"The bank alerted that it would change the way it carries out auctions," said Reginaldo Galhardo, a currency trader at Treviso brokerage in Sao Paulo. For a market that had seen the central bank on the sidelines, "this said to traders 'You think we're not watching, but we see what's going on.' Some in the market backed off thinking that the statement was an actual auction announcement."

The central bank said in the statement that it would accept bids during auctions via a new messaging system, part of its upgrade of communications with investors.

Traders also noted that the central bank said the new system would also apply to spot market intervention, reminding traders that it could also resume interventions in the spot market after focusing mainly on futures markets in recent months, Mr. Galhardo said.

The view that the government would allow the currency to weaken further wasn't unanimous, however, with Barclays writing that "the ceiling is closer to 2.10 than it is to 2.15."

"We expect the BCB not to sanction such a change in range in the last few days of November," Barclays economist Guilherme Loureiro wrote. "There are US$3.14 billion in derivatives coming due at the end of the month (BCB is long USD) and we believe a new (higher) trading range should only be confirmed if it decides to roll that exposure."

Likewise, Bank of America Merrill Lynch economists wrote that "there is no evidence the central bank has changed its intervention policy," and attributing the lack of recent central bank auction to lower volatility than in previous months before, noting that "a much weaker currency will clearly add noise (and pressures) to an inflation rate that is already inching higher."

Still, some investors say that the real's record strength against the dollar in recent years -- the currency peaked at BRL1.53 per dollar last year -- is a thing of the past.

Luis Stuhlberger, co-founder and chief investment officer of one of Brazil's largest asset managers, Credit Suisse Hedging-Griffo, said Tuesday that current weaker levels for the Brazilian real are likely here to stay.

Given the reduction of short-term interest rates in Brazil, the dollar likely won't fall below BRL2 again "in our lifetime," Mr. Stuhlberger said.

Write to Paulo Winterstein at paulo.winterstein@dowjones.com