Re: Brake master cylinder

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

> From: Chris & Ranae Procyk <procyk19@domain.elided>
<snip> 
> On all mixed-brake vehicles I have ever seen, the front discs have a
bigger res. chamber than
> the rear drums  Is just the chamber bigger, or is the bore (and therefore
the fluid displacement per
> stroke) bigger too?  It might be-- which would explain why the front res.
is also always at the rear
> of the cylinder...can't machine it otherwise.

As a compensation for pad wear, disc brakes surely require a larger
reservoir -- owing to the larger diameter of the caliper piston.  The self
adjusters in drum brakes keep the wheel cylinder pistons operating in
approximately the same area of the wheel cylinder bore throughout the
wear-life of the shoes.  No additional brake fluid needs to "migrate" from
the master cylinder to the wheel cylinders over the life of the shoes.
Contrast this to the discs (with no self-adjustment), whose pistons operate
further & further OUT of the caliper bore (closer to the rotor) as the pads
wear. A larger reservoir (or more frequent checking) is required in order
to assure an adequate fluid supply as the pads wear and the fluid volume
behind the caliper piston increases.

But as for fluid displacement requirements *per application* of the brakes
(on each stroke), it's my sense that discs would require LESS fluid than
drums.  Discs don't "retract" after each application like drums do. 
Therefore, every bit of fluid forced behind the caliper piston "builds
application pressure".  A certain amount of fluid in the drum brake wheel
cylinders is required just to "displace" the shoes, overcome the springs,
and make the shoes contact the drums at all.

Bill Thebert
The Binder Bulletin
(whose degrees are in philosophy and fine arts, and NOT engineering, and
who could therefore be all wet)