My 95 has a typical non-ABS master cylinder consisting of two
hydralic circuits.

When applying a light touch to the brakes at a stop (or when creaping
forward), the brake pedal sometimes gives and goes farther to the
floor than is normal. Coming off the pedal and reapplying firms up
the pedal.

The pedal holds firm if one applys a hard stop and does not come off
the brake.

Does this indicate that the master cylinder is failing?

I would think that a master cylinder failure would typically effect
just one of the circuits and would express itself by a pulling to the
side when braking.

Or will the pressure regulator valve mask the symptom by allowing one
circuit to bleed over to the other circuit?

Here is some more info:

There is no loss of brake fluid. There is no brake malfunction
lights.

Air in lines, worn brakes or master cilinder are probably it

<kiselink (AT) mindspring (DOT) com> wrote

I think answering brake questions worries some folks as you're talking about
a 'life and death' thing here. So with the over-riding proviso that you
should seek professional advice on things like brakes, I'll offer my
observations on brake problems I have encountered. My list is by no means
complete or I suspect comprehensive compared to a mechanic who deals with
brake-problems everyday,...so here are my limited comments:


IME, a pedal which 'pumps-up' can mean there is air in the system
(brake-bleed req here) or there is too much clearance between the rear-shoes
and their drum. On my car the rear drums have a selfadjuster which is
supposed to keep lining-drum clearance correct,..but these for some reason
often dont work all that well.

Other faults I've heard of include: spongey, cracked, ballooning, hydraulic
hoses (between the metal brake-lines and the road-wheels)



This is a pleasing sign ( the pedal not sinking to the floor) but to test
for by-passing seals, a more moderate pressure is applied which is continued
for minutes. Any sinking to the floor is a sign the mastewr-cyl needs
overhauling or depending on the condition of its bore, replaced.

The faults I've encountered and their causes are:

The MC leaking fluid down the front of the booster (which indicates a
leaking rear-seal)- pedal sinks slowly to the floor, (bypassing seals due
wear and/or a scored-bore) - causing brake drag (faulty residual pressure
valves),..but I'm sure there are others.


No,...the dual system duplicates the fronts wheels and one rear-wheel. So if
one system fails, you may not even be aware of it.


Cant answer that as I dont know what you mean.

I'd get a w/shop manual, and follow their 'fault symptoms and remedial
action' section. If you think things are still not OK,..spend the bucks at
the brake-shop!!

NB

The power-brakes booster can mask some MC/ brake faults. It can be checked
by turning the engine off, pump the brakes 5 times (to deplete any
vac-resevoir) the while holding the pedal down with your foot, start the
engine. The pedal should sink slightly as the engine applies vacuum to the
booster's diaphragm. This test IMO, should only be regarded as a basic test.

I once had a brake problem in a Ford V8 where the booster would lose action
while I was creeping along in traffic (very disconcerting!). This turned out
(after a long time of hit and miss diagnosing) to be a faulty PCV valve
which was robbing the booster of vacuum 'top-up' because the valve was
allowing too much crankcase ventilation at idle and the fact it was
positioned on the same plastic intake-manifold access tree. This why I say
an expert is needed in many cases with brake problems.

Jason

You say the pedal holds firm if you press firmly on the brake pedal and
hold.
If the master cylinder had internal leakage past the seals, which is
how they "fail", then firmly pressing on the pedal would cause the
pedal to sink gradually, so sounds like the master cylinder is OK.
Try having the brakes bled properly. That would be the first step to
removing air in the lines which can also give a soft pedal.
Maybe have them bleed the brakes twice. Once they get it set up, the
second time is fairly easy, and most of your cost is in labor, so the
small amount of extra fluid is only around $5 for the second time.
That way you're sure you've got fresh fluid throughout the system, and
that it is completely free of any hidden air pockets.
If you still have a problem, then you can investigate further, but I
suspect that will cure it, and is a good general maintenance procedure
in any event.

Yep - thats what I am doing. I know that there are pros here! Are
you one?

Besides, I want to understand the physics so that I am not ripped off
by one of those supposed pros!
b-)

My camry has disk brakes all the way around ... so it aint that.

But won't you only get that symptom if the seals for both circuits
have failed? So then you need a double failure in order for that
test to work?!

I'll check that but if there is a leak - its so slow that the fluid
level is undetectable.

Didn't know that. But there are two circuits that come out of the
master cylinder. Its a standard simple master cylinder!

Unfortunately, the Camry factory service manual doesn't explain (or
even show) the layout of the brake system. Based on my reading
knowledge, generally, some cars use a left front/right rear and a
right front/left rear circuit split. Others work as you explain.

Typically, each circuit is directed back to a regulator value that is
somewhere else in the car. The regulator serves two functions. First,
it splits the circuit pressure so that the rear wheels get less
pressure than the front wheels. Additionally, the regulator monitors
the pressure between the two circuits and if there is a difference,
the brake malfunction light is activated. Whoops, I miss spoke - this
vehicle doesnt have one of these. Instead:

In this camry, there is a load sensiing proportioning valve (and no
regulator). It is in the rear and monitors the amount of weight in
the car by measuring the amount displacement of the axle relative to
the body. Based on the amount of displacement, different amounts of
pressure are given to the rear. (e.g., in a lightly loaded vehicle -
more braking occurs on the front).

I got the factory service manual but it doesnt have much in
diagnostics. The closest thing is "low pedal or spongy pedal" which
suggests servicing up to 6 different issues (including air in line or
master cylinder). But it does not provide guidance on distinguishing
the malfunctions.


Thanks much - for your suggestions!

On 28 Mar 2005 06:04:33 -0800, "Daniel" <nospampls2002 (AT) yahoo (DOT) com>
wrote:

I will try that!

<kiselink (AT) mindspring (DOT) com> wrote

Nope.


The physics of how a MC works involves seals, springs and valves. Diagrams
are needed to understand exactly how they function.


Yes.


So then you need a double failure in order for that
Yes,..my terminology sucks. I meant to indicate any slow downward travel
after applying moderate pressure over minutes, is not good. The reason I
said "moderate pressure" is because high pedal-pressure can make an
unacceptabley worn cup seal, regain complete sealing as its lip is forced
against the cyl bore.


I was refering to a dual safety circuit, not front vs rear.


Once again, that is refering to the 'front-rear' situation.


Jason