Narrative:

We were instructed to taxi the runway and hold short of the high speed. We complied with this; just taking a slow taxi down the taxiway; with number two engine in feather. After a 757 cleared in front of us on the high speed taxiway we were instructed to continue for the runway. We began to comply with ATC instructions. Upon reaching the taxiway intersection I began to apply aircraft brakes. We were still taxiing slowly with number two engine in feather. Upon applying aircraft brakes I noticed that we were not slowing down; even with brakes fully depressed. I then informed first officer of situation and he instinctively applied aircraft brakes on his side of the aircraft. His brakes did not work either. The first officer and I then applied the brakes together (you could feel them grab a little) and finally the aircraft came to a slow; gradual stop. I then set the parking brake. Sat there for a minute. Took a look at all hydraulic pressures; everything looked good. Sat for awhile longer. Then took number two engine out of feather. Hydraulic pressures still looked good. We then decided to slowly inch aircraft forward and test the brakes. When we did this aircraft brakes worked normal on both sides. We just sat there; called dispatch [and] maintenance. After discussion with first officer and maintenance; I elected to get aircraft towed back to the gate for inspection due to the possibility of a faulty brake system. Coordination was made to get a tug and maintenance personnel out to the our aircraft on the taxiway. The tug showed up first along with airport operation vehicle first.small air bubble was detected by maintenance in hydraulic system. Besides that everything was found to be normal.

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Original NASA ASRS Text

Title: SF340 Captain experiences brake failure during taxi out for takeoff. Aircraft is stopped when both pilots apply firm braking and then is towed back to the gate by maintenance. An air bubble is discovered in the brake lines by maintenance.

Narrative: We were instructed to taxi the runway and hold short of the high speed. We complied with this; just taking a slow taxi down the taxiway; with number two engine in feather. After a 757 cleared in front of us on the high speed taxiway we were instructed to continue for the runway. We began to comply with ATC instructions. Upon reaching the taxiway intersection I began to apply aircraft brakes. We were still taxiing slowly with number two engine in feather. Upon applying aircraft brakes I noticed that we were not slowing down; even with brakes fully depressed. I then informed First Officer of situation and he instinctively applied aircraft brakes on his side of the aircraft. His brakes did not work either. The First Officer and I then applied the brakes together (you could feel them grab a little) and finally the aircraft came to a slow; gradual stop. I then set the parking brake. Sat there for a minute. Took a look at all hydraulic pressures; everything looked good. Sat for awhile longer. Then took number two engine out of feather. Hydraulic pressures still looked good. We then decided to slowly inch aircraft forward and test the brakes. When we did this aircraft brakes worked normal on both sides. We just sat there; called Dispatch [and] Maintenance. After discussion with First Officer and Maintenance; I elected to get aircraft towed back to the gate for inspection due to the possibility of a faulty brake system. Coordination was made to get a tug and Maintenance personnel out to the our aircraft on the taxiway. The tug showed up first along with Airport Operation vehicle first.Small air bubble was detected by Maintenance in hydraulic system. Besides that everything was found to be normal.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.