7 posts before you by Green Dot:
the centre tank override jettison (OJ) pumps are switched off manually at approx. 800kg of fuel remaining in the centre tank. Float-operated shutoff valves prevent fuel scavenge when the main tanks are full. When the main tanks reduce to y-thousand the float-operated shutoff valves open and the fuel scavenge jet pumps will start scavenging (operated by motive flow from the boost pumps) the remaining fuel from the centre tank. The centre tank OJ pumps remain switched off.