Re: Dan's Master cylinder

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

> From: Daniel Nees <cookiedan@domain.elided>
> I was refering to Tom M's doubled up gasket. I think his
> doubled up gasket would prevent the gasket from expanding like it is
> made to do to keep pressure on the top surface of the fluid.

As always, I could be dead wrong -- but it's never been my impression that
this gasket applies "pressure" to the top surface of the brake fluid in the
reservoirs.

Rather, as the disk brake pads wear and the fluid level inside the master
cylinder drops, I should think that above the fluid becomes a LOW pressure
zone.  The normal ambient air pressure above the gasket (provided air can
"leak" through or around the metal cap to get ABOVE the rubber gasket) will
deflect the gasket downward.

It has always been my impression that the purpose of this gasket was to
reduce the exposure of the brake fluid to air, and nothing more.  Brake
fluid, as we know, is "hydro-philic" (loves to absorb water), and will do
so out of the air if given the opportunity.

A leak-free cap on the master cylinder will create a progressively greater
low-pressure differential above the fluid.  If the "leaking" can at least
be controlled and kept above the gasket, this additional air penetration
won't bring additional water to be absorbed into the brake fluid.  I can't
see how the presence or absence of a gasket either increases or decreases
the air pressure above the brake fluid.  It would appear to be dictated
solely by the amount of air infiltration INTO the reservoir as the fluid
level drops.

All that said, once the piston in the master cylinder is depressed PAST the
point of drawing additional fresh fluid from the reservoir, the master
cylinder is part of a sealed system extending down to the wheels.  What
goes on "upstream" from that point (above the fluid) shouldn't affect this
sealed system's ability (or lack thereof) to build pressure.  Only when the
master cylinder piston returns to the "back" position is it re-exposed to
the reservoir, for another little "gulp" of fluid -- as needed due to pad
wear, etc.

Can someone point out my misconceptions, if any?

Bill Thebert