It was the second consecutive delay, after stormy weather postponed a launch attempt early on Tuesday. Discovery and its seven-member crew were preparing for a 13-day supply mission to the International Space Station.
The valve problem surfaced as technicians began fueling up the shuttle for a launch that had been scheduled for 1:10 a.m. EDT (0510 GMT) from the Kennedy Space Center in Florida.
There was no immediate decision on when the next launch attempt would be scheduled.
Discovery and its seven astronauts were set to deliver lab equipment, supplies and spare parts to the space station.
The mission is among the last five or six flights devoted to resupplying the space station, a $100 billion project of 16 nations that is nearing completion after more than a decade of construction 220 miles (350 km) above Earth.
