CAPE CANAVERAL, Fla. (AP) — Spacewalking astronauts have topped off a leaky ammonia line but ran into trouble while trying to lubricate a critical joint in the power system of the International Space Station.

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An astronaut lost a bolt and got a washer stuck in a crevice. Mission Control told astronaut Mike Fincke he might have to settle for a partial lube job.

NASA wants to lubricate as much of the joint as possible to keep it functioning properly in the years ahead without any shuttle visits.

Ammonia is extremely hazardous, and the two did their best to avoid contaminating their spacesuits while replenishing the system with about 5 pounds of the substance.

It was the second of four planned spacewalks during Endeavour's final trip to the space station.