Simple Sizing Problem

[MadBojo]

Hello,

I'm fairly new to WPF, so please excuse my ignorance.  I'm trying to achieve two working areas for my program.  On the left should be a pinned tab which can be pinned/unpinned as needed.  The rest of the area should be taken up by a content area. 

I've achieved this with the following code:

<telerik:RadDocking Grid.Row="1" HasDocumentHost="False" >

            <telerik:RadSplitContainer telerik:RadDocking.DockState="DockedLeft" >

                <telerik:RadPaneGroup Name="SourceGroup">

                    <telerik:RadPane  Header="Sources" ContextMenuTemplate="{x:Null}" CanFloat="False" CanUserClose="False" IsPinned="True" />

                </telerik:RadPaneGroup>

                <telerik:RadPaneGroup >

                    <telerik:RadPane  Header="Data" ContextMenuTemplate="{x:Null}" CanFloat="False" CanUserClose="False" IsPinned="True" CanUserPin="False" />

                </telerik:RadPaneGroup>

            </telerik:RadSplitContainer>

        </telerik:RadDocking>

I want to keep the "Sources" pinnable pane to either a relative or absolute maximum size.  The content seems to break when I resize the window.  I've tried setting the MaxWidth on the RadPaneGroup and setting the HorizontalAlignment = "Left", but either way weird effects are produced.  I've included pictures of the two behaviors.  Thanks for any help you can give.

[MadBojo]

Is there more information I can supply to help get my issue solved?