Hit by more valve trouble, NASA postponed the launch of space shuttle Discovery just hours before it was to head to the international space station Wednesday because of a hydrogen gas leak. The potentially catastrophic leak was in a different part of the system that already had caused a vexing one-month delay. Shuttle managers put off the launch until at least Sunday and indicated that Monday might be more likely. The latest delay means Discovery's two-week flight must be shortened and some spacewalks cut out of the mission. That's because Discovery needs to be gone from the space station before a Russian Soyuz rocket blasts off March 26 with a fresh station crew. If Discovery isn't flying by Tuesday, it will have to wait until April. That almost certainly would bump the succeeding space shuttle missions as well as plans to double the size of the space station crew at the end of May, said Mike Moses, chairman of the mission management team.