QGraphicsView transparent over another QGraphicsView

I know this subject is frequent on forums but I can’t find a solution to my specific problem.

I’ve got a QMAinWindow, which centralWiget is a QMdiArea composed by several QGraphicsView.
I want to display a QGraphicsView over this QMainWidow and it’s QGraphicsView s.
I set the MainWindow as parent of my new QGraphicsView and apply a transparent
brush on it ( setBackgroundBrush(Qt::transparent); viewport()->setAutoFillBackground(false); scene->setBackgroundBrush(Qt::transparent); )

The result : the new QGraphicsView is not transparent when it passes over a QGraphicsView from the QMidAre of the MainWindow. If there’s no QGraphicsView, there’s no problem it’s transparent (I’ve got a pixmap for background of my mainWaindow and it appears)

If someone knows how to fix this problem it would be welcome
Thanks