Brake pedal goes to floor

SJ syljay at
Thu May 6 11:28:18 EDT 2004

From: "John Larson" <j.d.larson at>
To: "SJ" <syljay at>; <quattro at>

> > What is the failure mode for the sudden collapse of  the MC?
> There are 2 common M/C failure modes in our cars.  In one, the brake
> pressurizes all by itsownself, causing the car to come to a stop and/or
> requiring copious amounts of power to overcome the drag.  In the other,
> pedal goes suddenly to the floor and you can't stop at all.  This mode may
> be intermittent, but it'll become increasingly frequent until you finally
> have to use a large fixed objet to halt your forward motion.   HTH, John

**** I was not clear with my question.
I wanted to know what happens inside the M/C to cause the "pedal to floor"
symptom. What breaks inside the M/C?

I'm assuming that the basic operation is similar to a hyperdermic needle
My guess is that the rubber end of the plunger breaks, or dissolves, or
folds over iteself, thus letting brake fluid slip past the rubber seal.

Another possibility would be rusted and pitted brake cylinder bore, which
would allow brake fluid to slip by the seal. But, this would cause a slow
sinking of the brake pedal . . .not the fast sinking that I have.


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