With a leaky piece of undersea equipment repaired, engineers were ready to begin a twice-delayed test of the leaking BP well to determine if the well's piping beneath the sea floor is strong enough to withstand a full seal from a new cap.

"We are ready now to restart the procedure," said Thad Allen, the retired U.S. Coast Guard admiral who is heading up the federal response to the Deepwater Horizon oil disaster.

The leak was discovered Wednesday night in one of the lines that diverts oil from the new, tight-fitting cap on top of the well.

Allen said at a news conference Thursday that the equipment had been repaired. That means the most important part of the test -- a measurement of the pressure inside the well -- probably will likely begin Thursday, after BP slowly closes the valves in the cap that are allowing oil to escape.

A BP spokesman would not pinpoint how far along engineers were with the valve closure procedure late Thursday morning.

Once the well is sealed at the top, experts will closely monitor pressure measurements. Low pressure may indicate that the well is leaking oil from other locations. If that is the case, the cap could exacerbate the flow from cracks in the well. In the worst-case scenario, oil could rise to the sea floor, creating new surface leaks.

BP officials said that if they detect low pressure, they will unseal the cap and revert to their oil collection plan, using a series of pipes leading to containment ships on the surface.

But if the pressure readings are consistently high for 48 hours -- meaning the well may be strong and intact -- there is a chance that BP will keep the cap completely sealed.

That decision will not be made immediately, however. After those 48 hours, Allen said, BP will go back to taking up the oil using the pipes and containment ships. Then, Allen said, a seismic test of the sea floor will be conducted "to see if there are any areas where pockets of oil may have formed as a result of being forced out of the well bore."

-- Richard Fausset in Atlanta