Inventory module Security

I need to create record level security for Inventory management module where the user must be able to view the records based on his role. I have created 2 user groups namely BUS access and Truck access. For BUS access i have given the constraint that it should display only Site no:1 for site specific order settings .Similarly i have given the same constraint for TRUCK access where the user will be able to see only Site no:2.

But when i open the Item master form and go to site specific order settings the user is able to see all the records in all the sites.

How do i resolve this issue??