Re: My master cylinder

["Bill Thebert" <bthebert@domain.elided>]

> From: Daniel Nees <cookiedan@domain.elided>
> Subject: My master cylinder

Quoting two separate sections of your post, Dan:

> As the fluid level gets low in the
> resivoir, the vacumn created pulls the gasket down into the resivoir. 

and

> But, if the surface of the fluid was pressurised,

Here's where you lose me.  Above the surface of the fluid, is there a
"vacuum"?  Or is it "pressurized"?  It feels like you're saying both, and
my sense of the world is that it has to be one or the other.

> This, I think, is because the
> wasn't any vacumn pressure on the surface of the fluid. 

Here, in regard to the Maverick, I don't understand the phrase "vacuum
pressure".  These seem like opposites to me.

On further reflection, let's assume that the Lid-gasket-reservoir seal is
perfect in all respects.  The master cylinder lis is not perfectly flat, so
there is *some* air above the gasket.  Let's also assume that you fill the
master cylinder perfectly brimming on a level surface, so that there's *no*
air UNDER the gasket.

We start with ambient pressure (+/- 15 psi above the gasket).

As the fluid level drops below the gasket, the air above can expand.  The
gasket is deflected downward.  The air above the gasket -- now occupying a
larger space -- is at something LESS than 15 psi.  As the fluid level
continues to fall, the air above the gasket continues to expand.  And the
"air pressure" above the gasket continues to fall.  

Even if the lid leaked and allowed air to enter above the gasket, it would
still never be MORE than the surrounding ambient air pressure.  And
surrounding ambient air pressure is precisely what would be acting on the
brake fluid surface with an open-topped master cylinder with no gasket at
all.

I'm not "denying" or arguing with your past experience, Dan.  I just don't
"get" it yet.

> You will get
> some fluid out of the resivoir on each pass of the piston, but not a
> full pistons worth. 

Don't think you *need* a "full piston's worth" on every stroke.  Return
springs on the wheel cylinders force fluid back into the master cylinder
piston bore.  Same with any elasticity in the flex lines or elsewhere in
the system.  I think one only needs a "tiny gulp" from the reservoir, to
make up for a small amount of disc pad or brake shoe wear each time. 
Otherwise, where would all that fluid be *going*?

Best Regards,

Bill Thebert