Some Design Master items are created by grouping multiple standard CAD entities. There are times when entities that are part of the item are not properly removed. Using the standard CAD ERASE command will erase the entire item. Use the Erase Part of Grouped Entities command instead to remove the entity that is no longer needed.
To erase part of an item, go to
Ribbon: DM HVAC->Drafting-> 
Erase Part of Grouped Entities
Pulldown Menu: DM HVAC->Drafting Tools->Erase Part of Grouped Entities
You will be prompted to specify the part of an item to be erased.
Select entity in group to erase:
The entity will be erased.