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

[jcd914]

It is possible to run the reservoir low while bleeding the brakes or if the pads were worn down a lot and get air bubble in the supply line. It is also possible to get the air bubble caught in the supply line in the semi flat section. What happens is that brake fluid flow around the air bubble while bleeding. In the flat section of the supply line the bubble clings to the top of the tube and the fluid flow under it. It does not quite flow under as fast as you are bleeding so the bubble moves down toward the master cylinder but does not quite make it as the supply tube angle get steeper. As soon as you stop the flow from bleeding the bubble moves back up as the fluid flows around it.
I have had good luck by slapping the brake pedal over and over again very fast. You want to create very short pressure pulses, the bubble will move back a forth very quickly but usually it will move up stream a little each time and will end up going back to the reservoir.
Since you have the tank out you may be able to just flex the tube and increase the angle so the bubble flows up and back to the reservoir.

good Luck

Jim