[MaSnaka]

Is it possible your pedal ratio is set up for manual brakes? That would be a significant increase in pedal pressure with less effort because the pedal would have more leverage on the rod. Maybe the combo of that and a booster is too much. The pedal box has the holes drilled for both set ups but the difference would be in the length of the pedal arm between fulcrum and the cleavis. I have a picture in my gallery showing the difference in pedal arm length. HTH.

John