Filtering a main form from a subform

[Stargrove]

I have a form that holds work order information. Each work order can potentially impact a group of systems. The systems information is a subform on the main work order form.
I need a way for a user to go in and set a filter on the subform so he only sees the work orders that affect a particular system. I am currently including the system information in the main query. I get many more records as part of that query (as expected), but I am able to set the filter using a pop-up form with a combo box. The current way I am doing it makes the data on the main form read only. Is there a better way to do this that will work?

Thanks,
James