Stand pipes
The kit I flew had stand pipes on the jettison pump feeds which were a fair bit above the bottom of the tanks so that you could only jettison down to a certain level.
There was an infamous incident ex LAX which landed in Manchester where the guys put out a mayday after the cross feeding checklist was changed to use the jettison pumps and the heavy crew erroneously believed they were going to loose a second and third donk of final approach due to fuel starvation. As it was there is a safety feature which allowed the tank with fuel to suction feed iirc.