I am an expert in expatriate taxation. This is an area that I have practiced in for over 30 years.
In Brazil, we do not have an income tax treaty. So this means that your SS will be taxable in both the US and in Brazil. It is US-sourced, so even though you are living in Brazil, it was earned in the US, and paid by the US government, so it is taxed in the US. Some of the tax treaties that we have with the various foreign countries allow this to be exempted in one country or the other. But we have no tax treaty with Brazil. You could try to file a Brazilian tax return and claim a credit for the US taxes paid, but if your $2,500 per month of SS is your only income, you probably won't pay much, if any, US income tax.
At $30,000 of income, you are in the 27.5% bracket for Brazilian taxes. They use a graduated system, from zero to 27.5%. They allow deductions for various items, so you won't pay tax on everything. The 27.5% bracket starts at about 54,000 Bs, or about $16,400.
I am guessing that you have a VIPER visa, so you have to put more than $2000 in a Brazilian bank account each month.
I hope this answers your question. If you have any more, please feel free to let me know and I will be happy to answer.
Have a great weekend!