Slalom4me

Is someone able to provide front & rear output
pressures at the ports for OEM C4 master cylinders
with the early (7/8") and late (15/16") piston bores?

If available, what was the pedal pressure and the
pedal ratio when the output pressure was recorded?

Thanks,
Ken R.

.

silver84

does anybody have a good explanation of what
the parts do in an early-C4 M.C.?

for example, why do there appear to be two
'floating' pistons, when only one is needed?

Slalom4me

This creates two isolated circuits for braking so that in the event
of a failure somewhere within one circuit, a portion of total braking is
still remains available through the other circuit.

How Master Cylinders and Combination Valves Work

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silver84

to do what you describe,
it would seem to need,
one 'drive' piston, plus a floating piston.

why two floaters?
what does the side chamber do?
how do the springs effect brake bias?
if one front is lost, does the other
front brake still work?

Slalom4me

silver84,

I believe that early C4's like the '84 had single reservoirs but
RockAuto currently displays the following image for the car.



In the m/c above there are three pistons: one 'drive' and two 'floaters'

The 'drive' and 'floater' pistons in the main bore of the master cylinder
are more commonly referred to as the Primary and Secondary Pistons.

The second 'floater' that I think you are referring to is in the side
chamber. The manual describes this as the Proportioning Valve (but I
would describe it as a Combination Valve because it also includes a
System Failure switch.)

The spring in a proportioning/combination valve affects bias by limiting
how much of the available pressure goes to the different circuits.
If there is a metering valve, this affects the timing of when pressure
begins to be applied.

The spring resists pressure trying to shift the proportioning valve. As
long as the spring pressure is greater than the force acting on the
valve, the metered port remains open. When spring pressure is overcome,
the valve shifts, closing the metered port and thus limiting further
increase in pressure on that side of the circuit. An adjustable
proportioning valve provides a means to manually change the spring
pressure.

I do not know the C4 plumbing well enough to be certain, but my vote
is it does not permit one front to remain active if the other one fails.
Whether this is the case and whether there are differences between
non-ABS/early ABS/late ABS cars I can not presently say. I know that
there used to (european?) cars that were promoted as being able to
retain 3 wheel braking in event of failure on one wheel.

For interesting reading, look at S7.9.3 & S 7.10.4 here

BTW, silver84 - do you happen to know the answer to my original question?

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silver84

Slalom', thanks for responding to my post.

the picture, looks exactly like the MC on my 84, two bottles

from time to time, I try to collect all the 'brake system'
info I can, maybe some day, we will know exactly
what all the brake specs are.

?one 'floater' splits front and back, the other
is concerned with reducing force to the back?

?the 'bias' floater, is in the side chamber?

do we know how much the force reduction is?
do we know the spring rates of either the stock
or the DRM replacement?

about your original question, sorry, don't know,
we need to get the attention of CentralCoaster.
.............................
OK, I just looked in my Haynes.
the 'combinatin piston' is in the ''''side chamber'''.

if we knew the amount of force reduction to the rear,
I think it would be helpful.

I'm guessing, the amount of force PSI reduction,
is proportional to the force on the spring?
........................
sorry if I haven't help much, not much tech info
here, mostly rehashed advertising.
for example, when I ask...does anyone have
a 'suspension analyzer file' for the C4 ? ,
nobody responds.