Well, I may be wrong. I just have some problems seeing PSDB as centre-left or simply centre. Maybe I'm not familiar enough with Brazilian context.
The military dictatorship had it's toll in the political thinking of the Brazilian society, in my opinion. As i understand, there's a bit of right-wing trauma where people on the right or liberal conservatives are a bit embarrassed to admit what they believe in. The same happened in Portugal after the 25th April and even today there's a bit anti-rightwing, anti-liberal speech here and people just don't say what they really think. That's one of the reasons the political parties names are so weird.
As i understand, by what Yankee said, the PSDB is the anti-PT party. It doesn't matter if they are center-left, center or center-right, they aggregate many groups of different political views that have one thing in common: they're distrust of PT. In politics, that's enough.
Also, i find it fascinating the similarities between the PSDB and the early PSD in the 70's. In 1976, the PSD also applied for the Socialist International but the PS block it. Because of this decision, Sá Carneiro, the founder of the PSD, swung to right and that shift to the right was cemented during the Cavaco Silva years between 1985 and 1995. Interesting.