I found the opposite. Fired up GT5 for the first time in I don't even know how long; at least 9 months. I found I was getting some rear-brake-lock-rotation in some cars. Caused a few clenchers, was good fun.
I really, really, really wish that GT5 had more customization options like Forza 4. Or I just wish that FM4 was for the PS3 as well, so I could play it. I wanna paint my car, damnit!
did you adjust the brake bias? The brakes have caused no trouble in the cars I've been using.
As a separate related issue when using better tires, the springs need to be stiffer to account for the extra grip on the brakes. If left the same the car will dive a lot causing those clenching situations.