because the capping unit on the pro7880 printers covers the whole head there's no way to just empty one line. newer Pro7900 can flush two lines at a time because the inking system and head is split 2 x5,
options are taking the line off at the damper assy on the head, taking the ink cart out and locking the leaver then draw out the ink manually
or
doing about five power cleans.
using a purge pattern will take forever as there's around 30ml of ink in the line, damper and head. one A3 at 280% ink is about 2.7ml of ink on paper.