duckbill wrote:
> Over the past few weeks the brake pedal on my 95 Civic Ex (195,000
> miles)with ABS would sometimes sink a little at stoplights and statrted
> feeling a little funny the first time I applied them lightly in the
> morning. My first thought was to bleed the brakes. And, while bleeding the
> brake pedal yesterday, I lost all pressure and the brake pedal consistently
> went to the floor. I had placed a piece of wood under the pedal so the
> brake stroke would be the same. The search feature of this wonderful forum
> led me to purchasing a master cylinder. The problem is now fixed.
> What I don't understand is how a master cylinder could fail so
> completely because you have two seperate hydraulic brake systems? When my
> pedal went to the foor, it meant I had lost both of these brake systems.
> There were no external leaks, and I never had to add brake fluid. Had one
> of my brake systems failed a while back and I was not aware of it? Thanks
> in advace of any thoughts on the issue.
>
the seals wear and leak. fluid passes past the seals - just like a
shock absorber. it's real simple. the floating secondary piston
doesn't make any difference to this as all seals usually wear at the
same rate. the only thing to worry about is replacing and bleeding,
which you have done, and taking the care to flush the system with fresh
brake fluid every year or two, depending on climate.

in the future, if you're bleeding a new master cylinder, don't dick
about with the piece of wood. the pistons are designed for full travel,
and failure to exercise that on some vehicles means you'll never succeed
in getting all the air out. the wood theory comes from old wives tales
about "i bled the system and the cylinder failed a couple of weeks
later". reality is, the seal was shot but swollen due to contaminated
fluid, therefore it kinda-sorta held in there. clean fluid re-shrank
the seal, which would have happened regardless of bleeding method, and
it started to leak. not changing the fluid may have allowed it to
survive a little longer, but it was on its way out anyway. moral of the
story: change the seals/cylinder and bleed properly. if you expose
additional problems, it's a good thing - this is a critical safety
system after all...