When we get all of the old sand out, we lift out the drains and remove the pea gravel surrounding them(pea gravel is the what the drains were originally surrounded with). When the drain is out, we flush the line to make sure there are no blockages.
When everything checks out OK, we put the drain back, but with a filter sock around the drain. The filter sock prevents debris from entering the slits in the perforated drain pipe. With the drain in the trench, we then backfill the trench with bunker sand instead of pea gravel. I saw this technique a couple years back when Shane Ferrill (Superintendent) was giving me a tour of Boulder Ridge. Thanks to Shane for another wonderful idea to add to my repertoire.