> Brent is probably right that a rapid stroke can break up an air bubble
> but what usually happens is that the air just moves a few inches along
> the pipe on the slow down stroke and then runs back to its original
> higher point again.
***************************
This is an excellent point. I've encountered exactly this phenomenon
with larger-diameter brake lines (3/16 inch ID or larger), in which case
many or most bubbles are smaller than the inside diameter of the tubing
and are able to climb more quickly back up to a high point during the
interval between pedal strokes.
I've never noticed it with smaller lines (such as 1/8-inch ID or less),
since most bubbles are large enough to fill the inside diameter of the
tube and act almost like pistons in a cylinder, trying to compress the
fluid ahead of them as they try to climb back upward. I suspect that the
additional area (relative to bubble size) in contact with the sides of
the tube (and other esoteric fluid dynamics that I don't pretend to
understand) would also play a part.
One other thing about smaller lines is that a pedal stroke of a given
speed and volume would create a higher velocity flow in a smaller line
than in a larger line, perhaps helping to purge the system of bubbles.
Alan
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