Air is the most common culprit so keep trying. Make sure the reservoir is full, could be air in the top end.
Does it get worse after a ride? Water in the fluid (its hydroscopic you know
), a 2005 bike should have had three fluid changes, two at a push. If it's never had one it'll be due to go off, the fluid will smell bad and be black/brown/green.
If all else fails or the fluid is discoloured buy a bottle and keep going with the open nipple -squeeze to the bar-close nipple-release cycle until you have fresh fluid and no bubbles.
If that doesn't do it, could be a seal somewhere.
Andy