Years ago I came across a MC that just wouldn't bleed.
And the harder I looked at it the more weird it got. I
didn't just seem to keep passing air, but rather
either bleed screw would expel out, then suck an equal
amount right back in, they would wheeze. After many
hours.... I took the MC off and dismantled it, every
thing was in tip to shape, bore OK, seals nice and
meaty... then I noticed the end, (or is the
beginning?) of the first plunger. (Let's just call it
the end.) The end of the first, leading plunger has
this casting on it, some MC's the casting is metal,
some they are plastic. This casting looks like the top
of a 'castle chess piece', and nestled down inside is
a small elastomer seal underlayed by a spring steel
wave washer. Well this casting was metal and the
leading edges of it had corroded away so that it was
flush with that elastomer seal. The leading edge is
supposed to stand proud of the seal when everything is
at rest. The way I understood it at the time was this.
When you step on the brake pedal and compress the MC
pistons, that casting bottoms out on the end of the MC
bore and then that seal is pushed forward still and
bottoms out too, and do you know what it does? It
covers the hole/passageway to the reservoir preventing
brake fluid from being pumped back into the reservoir
while you are trying to apply pressure to your brake
system. Pretty neat hey?
So maybe the casting is worn or the seal is worn.
Before that episode, I never understood what the
function of that seal and casting was.
Steve J. Daniels.
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