Air carrier First Officer reported a critical ground conflict during final approach with a departing aircraft on the runway. ATC instructed the flight to go around and they returned for another approach and landed safely.

Narrative

In and out of IMC on descent and approach. When we extended the gear and continued lowering flaps to 30 at G/S intercept on our initial approach via ILS26R; we received a brief EICAS message and the associated aural tones for C HYD DEM PUMP. The message disappeared as quickly as it had appeared. At the time; we planned to write it up once we landed; as we believed it related to a previous write-up in the Maintenance LOG that had been closed.As we approached 500' AGL on G/S; we were directed to execute a MA by ATC due to departing traffic remaining on the runway. As we executed the G/A procedure; the previous EICAS message reappeared intermittently while cleaning up to Flaps 5.The Relief Pilot referenced the FM while the CA and I remained busy with quickly changing vector; altitude; and frequency assignments from ATC. He pointed out that the checklist NOTE required checklist execution in the event of momentary message display. On base leg; with the AP now engaged; the CA was able to consider/execute the checklist steps. At step 2 of the checklist; the PRESS light remained illuminated. Given the associated notes thereafter; the CA asked me to [request priority assistance]. [ATC was advised] shortly before receiving a dogleg vector and approach clearance. We were subsequently passed to Tower; received landing clearance; and landed uneventfully. The AP disconnected (uncommanded) prior to G/S intercept during this approach.Due to our position and the critical phase of flight at point of decision to declare; the CA complied with FOM XX.XX.X requirements for [priority assistance requested] once at the gate with engines shut down.