Depends on how much you pay for 7800, if its a couple of hundred dollars then possibly worth the effort to get working again, I'd buy some Inkjet mall piezoflush and empty carts and flush the machine, paying particular attemtion to any clumping in the ink delivery tubes that run from the ink carts to the head assembly, before trying to clean the head, if the ink has clumping then you are just forcing the clumps into the (new) dampers and printhead, use a syringe and disconnect each ink tube and flush existing ink out first.
Bob.