Launch officials waited almost an hour after the leak appeared during fueling, trying to fix it through remote commands, before calling off Wednesday's pre-dawn launch.
The leak occurred in the same place as one that cropped up Saturday: in the hydrogen gas vent line that hooks up to the external fuel tank. A similar problem stalled a shuttle flight three months ago.
"We're going to step back and figure out what the problem is and go fix it," said deputy space shuttle program manager LeRoy Cain. "And then we'll fly as soon as we're ready to safely go do that."
The baffling leak is about the size of the point of a ball point pen. Workers on Saturday tried repairing it, but it didn't take. They were able to repair the problem on the March mission, but NASA still doesn't know its cause. Cain said an engineering investigation would find out what's causing the problem.
Launch officials said they were proud of the way the team hustled over the past four days trying to get Endeavour to the space station, with a new outdoor addition for the Japanese lab.
"I sure wish we could have rewarded them and the astronauts and everybody else with a launch this morning," assistant launch director Mike Leinbach said. "But the leak was way out of spec again, and so we were just not comfortable pressing on."
Even before hydrogen gas began leaking — a serious situation because of its flammability — NASA was up against a tight deadline for making the 5:40 a.m. ET launch. Fueling was delayed three hours by thunderstorms Tuesday night.
The seven astronauts were still in crew quarters when the leak was detected.
"I'm sure you all know that we postponed again," commander Mark Polansky wrote in a Twitter update. "It's a reminder that spaceflight is NOT routine. We will fly home to Houston this morning."
Noted Cain: "This business that we're in is not for the faint of heart."
NASA bumped an unmanned moon shot — its first in a decade — to give Endeavour this second chance of flying before a thermal blackout period kicks in.
That moon mission, featuring two science probes, is now scheduled for a Thursday or Friday launch. After Saturday, unfavorable sun angles prevent Endeavour from taking off before July 11.
Cain said it was too soon to know whether NASA would be able to make a July 11 launch.
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Endeavour was set to deliver the third and last segment of Japan's massive space station lab along with hundreds of pounds of food for the six space station occupants. A new space station resident also was supposed to go up and swap places with a Japanese astronaut who's been up there since March.
The space station crew doubled in size late last month.
When Endeavour finally flies, it will be one of the longer international space station visits — nearly two weeks docked at the orbiting outpost — and include five spacewalks.
Once the shuttle pulls up at the space station, there will be 13 people together in space for the first time ever.
Delaying until July is expected to push back the next few shuttle flights.
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