Thanks, now i got it right. I set custom bounds that the Y-bound is just 0.99 and not 1. So u go into the block and the method is triggered.
Here is my solution
@Override
public AxisAlignedBB getCollisionBoundingBoxFromPool(World par1World, int x, int y, int z) {
return AxisAlignedBB.getBoundingBox(x, y, z, x+1, (double) y + 0.99 , z+1);
}