Brakes Problem?!, Air bubble in line from reservoir... Options

[Nor.Cal.914]

While in the process of doing a brake bleed I noticed an air bubble in one of the lines from the master cylinder reservoir. I have my gas tank pulled right now so I am able to see the lines all the way down through the body where they meet with the master cylinder. The bubble is roughly 1/4 to 1/2 inch in length and sits right in the flat spot before the lines bend downwards to pass through the body into the master cylinder. To try and fix the problem, we used a vacuum bleeder to try and create vacuum at the left front caliper. (I'm using the left front since it is the shortest distance from the master cylinder) I then pumped the pedal up after there was a bit of vacuum in the bleeder and opened the bleeder screw. The bubble moves down the line towards the master cylinder when the bleeder screw is open, but once the screw is closed again and I pump the pedal again the bubble comes right back to where it was before. I also tried just normal bleeding of the brakes by pumping the pedal and opening the bleeder screws, but the bubble doesn't even move when I bleed them the normal way. Also, when I sit and pump the pedal up and down the bubble moves back and forth probably an inch in total. And when I do the normal bleeding of all the calipers there is a constant flow of fluid; no air at all (IMG:style_emoticons/default/confused24.gif)

Soooo... does this mean my master cylinder is allowing air to come back into my lines? Would it be an internal leak possibly between one of the pistons? I'm not exactly sure how the master cylinder comes apart on these cars, except for what I'm shown in my manual. Could it be that the two lines coming from the reservoir are leaking at the point where they meet the top of the master cylinder via those two rubber "plugs"? Something has to be allowing air back in making it impossible to remove that bubble! Anyways, let me know what you guys think (IMG:style_emoticons/default/sad.gif) Thanks
-Chris