Re: What's the rubber thing for?

[Doug Rasmussen <DougRasmussen@domain.elided>]

Message text written by "Tom Harais"
>That rubber "thingie" serves a couple of functions.  It acts as a gasket
between the top of the MC and the cover.  This keeps brake fluid in and
debri out.  Just as importantly, it keeps circulating air from contacting
the brake fluid.  VERY IMPORTANT. Brake fluid is hygroscopic - it attracts
water.  It will, and does, abosrb moisture right out of the air.  Water
boils at 212 degrees F.  DOT 3 or 4 brake fluid boils at a much higher
temperature.  Water in brake fluid causes problems.  Corrosion of the brake
system; A much lower boiling point for the fluid which means vapor and
compressible gasses in the system, leading to loss of brake funciton.

I think it would be much simpler and easier for you to get an appropriate
front disc brake master cylinder and replace your current one.

But, before you do, I suggest that you check your brakes system, especially
the wheel cylinders, for leaks.  If your brake fluid is dropping that
rapidly, I don't think it's from wear on the pads.  What kind of mileage
are you seeing between m/c "top ups"?<

Good explanation, Tom.  

Mostly, I thought it was for prevention of leakage and an anti-sloshing
device.  As I said my Toyota's don't have this type of rubber thing, but on
closer examination, I notice they have a little biscuit that floats on the
brake fluid.  Now I see that "biscuit" serves two purposes, to seal the
fluid from air and also as an electronic warning of low level.

Get an appropriate MC for my front discs?  From what vehicle?  Given the
tools I have available, machining an extension reservoir would be easier
than trying to adapt something else.

No, I'm positive there aren't any leaks in the brake system.  The first
time I lost pedal was right after the disc conversion, including umpteen
bleed jobs too.  I just wasn't paying attention.  That time the pedal went
all the way to the floor (partially due to lack of adjustment on the new
SII rear drums).   Yesterday's event is after, maybe, 3000 miles on the new
pads.  But I didn't lose all pedal, it was obivously less pedal than I had
gotten used to, though.  I immediately pulled over and  found at least 1/4"
of fluid still in reservoir.  A fill up brought full pedal back.   

Considering the differences in volume between calipers and wheel cylinders
none of this surprises me.  Approximately 2" dia caliper versus less than
1" dia wheel cylinder.......caliper has roughly 4 times the volume of the
original wheel cylinder.  A stroke of the brake pedal pulls a fair amount
out of that reservoir.

Thanks,

Doug