Transparency drawing brush problem

I have perspective camera in helix 3d view and i assigned solidcolorbrush with materialhelper behind my object which is a modelvisaul3D. I assigned a drawing brush with transparent background but I have problem with transparency.When i want to draw modelvisual3D with transparent background on the modelvisual3D with solidcolorbrush, but My problem is about Order of drawing this in helixviewer! Is there any problem with wpf or helix with this?

I think this link can tell my problem better =>
http://m.blog.csdn.net/blog/wmjcom/4038384