A similar leak occurred during the countdown for the shuttle Discovery in March.

“It was eerily the same,” Michael D. Leinbach, the shuttle launch director, said at a news conference.

Image The space shuttle Endeavour, which was to bring up the final pieces to complete the Japanese laboratory on the space station, will launch possibly June 20 or mid-July. Credit... John Raoux/Associated Press

Replacing the seals around the valve delayed the Discovery launching by four days, and the Endeavour launching will also be delayed by at least as long.

Mission managers had originally said that if the Endeavour could not lift off by Monday, launching would be pushed back to July because the Lunar Reconnaissance Orbiter is scheduled to take off on Wednesday.

Michael P. Moses, chairman of the mission management team, said there would be discussions with the managers of the orbiter mission to see if both launchings could fit into a small window in the coming week. “We’re going to have those negotiations,” Mr. Moses said.

If the lunar orbiter lifted off as scheduled, the shuttle could possibly depart on June 20. The need to prepare the launching area between takeoffs prevents making an effort sooner. Alternately, the shuttle could aim for Wednesday and push back the orbiter launching to June 20. “Nothing is a foregone decision,” Mr. Moses said.