I can't comment on compressibility of fluid with large bubbles versus micro bubbles of air, but I do know that brake fluid does , or can, have air dissolved in it.
Not to be confused with bubbles, which is entrapped air.

Alistair

> On May 1, 2014, at 7:23 AM, Gnarlodious <gnarlodious@GMAIL.COM> wrote:
> 
> One big advantage of the vacuum method is that microscopic bubbles
> expand under vacuum, causing them to flocculate faster and rise to the
> top sooner. I've noticed with pressure bleeding it can take some time
> to get microscopic air out of the system. If you watch fluid through a
> clear tube you see cloudy brake fluid coming out under pressure, but
> that just means microscopic air bubbles are incorporated in the fluid.
> 
> As a wild guess, I would say that one large bubble has more
> compressibility than a million small bubbles totaling the same volume
> of air. That would explain why your Vanagon's brakes feel fine
> immediately after bleeding but a few days later they are all squishy.
> This has happened to me several times and is something of a mystery.
> Someone more versed in physics could dig up the facts, I nominate
> Alistair Bell.
> 
> -- Gnarlie
> 
> 
> 
>> On Thu, May 1, 2014 at 7:38 AM, Mike B <mbucchino@charter.net> wrote:
>> Regarding the brake bleeding process. It all depends on you method used.
>>   If you use a Motive pressure bleeder, it's a simple one-man process,
>> and it's hard to "do it wrong".
>>   If you use a vacuum hand-held, hand operated brake suction device,
>> you need to grease the threads of each bleeder to prevent suction of air
>> through the threads, but otherwise, it's also pretty hard to "do it wrong".
>>   If you do it the old-fashioned 2-man way, there's a way to do it
>> wrong and a way to do it right.  The right way to do it is to attach an
>> 18" rubber hose to the bleeder and put it in a small plastic or glass
>> jar with a inch or so of brake fluid in it. (you could also buy a '1-man
>> brake bleeder' tool, which is basically this).  One guy does the pump
>> and hold on the pedal, while the other guy opens the bleeder and watches
>> the fluid/ air squirting into the jar.  Repeat on each wheel until
>> there's no more bubbles, being sure to stop every so often to check that
>> the reservoir isn't getting too low.  Use 3 sharp quick pedal strokes to
>> force air from all of it's hiding places, then hold until instructed to
>> let go by the guy manning the bleeders.  Start at the farthest away
>> (right rear), then move to the next farthest bleeder (left rear), then
>> right front, then left front.  You may need to go around them two or
>> three times to get it all, and a hard pedal with no squishiness is an
>> indicator that you're done. Be sure to do enough to clean all of the old
>> dark fluid out and replace it with fresh clear fluid.
>>  If you do it without the jar and hose, you will make a mess of
>> spilled fluid everywhere (it eats paint!), have lots of problems (like
>> Don mentions) and depending on your methods and procedures, you may not
>> be able to get every last bubble of air out of the system.
>> 
>> Mike B.
>> 
>> 
>>> On 5/1/2014 9:17 AM, Don Hanson wrote:
>>> 
>>>   Squshy brakes:   Bleeding the system carefully should stop that.
>>> "Carefully" being the operative word...Brake bleeding is intolerant of
>>> sloppy or slipshod work....any lapse at all during the brake bleeding
>>> process can render the whole procedure a waste of time...It can be a
>>> tedious job, relatively simple in concept but one that demands you make no
>>> mistakes along the way...It takes a fraction of a second of inattention to
>>> allow a bit of air to enter the hydraulics...a very very small bubble of
>>> air anywhere in the system will cause the spongy brake pedal feel..
>>> 
>>>    Another symptom you may be encountering here is warped rotors,
>>> oscillating as the wheel turns, a wobble that pushs the brake pistons way
>>> back into calipers.   So rather than an actual 'soft pedal' from air in
>>> the
>>> system, you may be needing to give the brakes a few pumps to move that
>>> brake pad back to where it will actually grip the warped rotor...