Here's an article on brakes I read awhile ago and found it very interesting
NSRA has recently added dual master cylinders to their list of not
required but neat things to have on your street rod from a safety
inspection standpoint.
While I do not dispute the fact that they are a great safety item, I
do have a problem with the blanket acceptance of the device as the
"end-all" to everyone's brake failure problems.
A lot of the following has to do with drum brakes as opposed to 4
wheel disc brakes.
Like most of you, I have been driving vehicles with dual master
cylinders for a number of years now. I don't think I have ever owned
(or leased -- I drove leased cars when I was in field service) that
didn't experience some kind of brake failure. Yea, the '81 Aries was
ok but I didn't have that one long enough to get into trouble. Same
goes for a couple of other short-mileage examples.
However, every vehicle I've owned/driven beyond the 100,000 mile mark
has experienced an hydraulic failure of some sort one or more times.
The ONLY times I totally lost my brakes were in dual master cylinder
equipped vehicles.
Why is that, you ask? (I've been saying that a lot in the past few
days) Let's take a look at driving habits and it'll become quite
clear why reality doesn't follow theory (sound theory, but incomplete)
I am not one of those individuals who spends a lot of time performing
deliberate forward/back self-sdjusting brake maneuvers. (better take
a minute and explain whatb that means for out novice readers.)
Self adjusting brakes on domestic vehicles employ a cable activated
ratchet mechanism to rotate a star wheel between the two brake shoes
in such a manner as to maintain a minimum clearance between the shoes
and drums. In order for this system to work, slack must be induced
into the cable to "reset" the ratchet. You will need to understand a
bit about self-energizing brakes to see how this works.
When you step on the brake pedal while the vehicle is in a forward
motion the wheel cylinder expands a pair of pistons out against the
two brake shoes. The front shoe snags, if you will, on the drum and
the whole front shoe / adjuster wheel / rear shoe assembly rotates
slightly in the direction of the wheel rotation.
Since the brake adjuster cable is a segment of the front shoe
mechanism, it goes slack and a spring on the ratchet at the bottom of
the front shoe drops the ratchet down as far as possible given the
confines of the system.
If, at this time, the driver comes to a complete stop and releases
the service brake (takes his foot off the pedal), the brake adjuster
ratchet is in the "cocked" position.
The operator then prceeds to back up and when the vehicle is rolling
at 3 or 4 mph the operator performs a firm and deliberate application
of the brakes. This causes the shoes and adjuster assembly to rotate
in the opposite direction. This reverse rotation of the assembly
STRETCHES the cable and actuates the adjuster ratchet.
If the shoes were far enough out of adjustment, enough motion would
be induced into the ratchet to cause the star wheel to rotate one or
more tooth positions thus lenghtening the distance between the bottoms
of the brake shoes (and closing the gap between the shoes and the
drum)
You have to go forward again and reset the ratchet if you want to get
another rotatation of the star wheel. You can't keep backing up and
hitting the brakes. They only adjust themselves on the first
application after a deliberate forward-motion stop.
If the brakes are propely adjusted there won't be enough movement in
the cable to cause the ratchet to increment the star wheel so once
they're adjusted, repeated forward/reverse won't over-adjust the
system.
Ok, now back to the problem. I, and a lot of other people don't, in
their normal driving mode, sufficiently adjust their drum brakes.
If the vehicle is 4 wheel drum, the brake pedal, over time, gradually
sinks to the floor. Something clicks in the brain and the brakes
are either serviced or the neighbors look at you in wonderment as go
forward and back in your driveway.
With a disc/drum combo, the failure to adjust the rear brakes isn't
anywhere near as apparent. We have to look at the basic master
cylinder design to figure that one out.
First off, the dual master cylinder is a pair of pistons not
mechanically connected to each other (unless there's a failure and
we'll get to that later)
The piston closest to the brake pedal has a pair of packings, both
facing forward, that cause brake fluid to flow when the pedal is
depressed.
When you step on the pedal fluid is forced by the rearmost packing
out into the brake lines to actuate the front brakes (they do 60% or
so of your braking, depending on vehicle weight displacement).
The frontmost packing forces brake fluid against an opposing packing
that's on the rear of the front piston. i.e hydraulic pressure within
the master cylinder is employed to actuate the piston controlling the
rear wheel cylinders. the front packing on this piston sends fluid out
to the rear wheels.
If there's a loss of pressure in the rear cylinder (the one controlling
the front brakes) the rear piston comes into physical contact with the
front piston 9the one controlling the rear brakes) and the pedal is
now mechanically connected to the rear brakes.
The problem with this scenario is that there isn't enough pedal travel
before you hit the carpet to get adequate application of out-of-
adjustment rear brakes.
With disc brakes, no appredciable amount of fluid flows back into the
master cylinder after application of the brakes so they're "self-
adjusting" and any hydraulic pressure increase has an immediate
effect on the application of the brakes.
With drum brakes, you have to re-fill the wheel cylinder each time
because the return springs on the shoes force the brake fluid back
into the reservoir.
That's why disc brake reservoirs are larger than drum brake reservoirs.
The drum fluid is "recycled" while the dusc fluid isn't.
So, HERE'S WHERE THE FALSE REASURANCE PROBLEM ARISES.
In most systems, the front brakes will more than adequately stop a
vehicle in normal driving conditions. The pedal resistance will be
pretty reassuring and even if there's a hole in thne rear brake line,
it won't be readily apparent because the pedal "feel" tells you
everything's OK. Not perfect, but OK. What WILL happen if there's a
hole in the line is the brake warning light will come on.
But let's say there isn't a hole in the line but the shoes are just
way out of adjustment. The spring pressure (brake return springs) is
going to present enough resistance against the switch in the
combination valve so that, UNDER NORMAL BRAKING, the light won't come
on telling you there's an imbalance in the braking systems. This can
go on for months and although you're still actuating the rear brakes
(and wearing them down) you're never ever lighting that lamp on the
dash that says you got a problem UNLESS you panic stop and really
bear dow on the pedal. THAT'LL get your attention.
Anyhow, I've lost the front grakes in my '72 Dart and, because the
rears were well enough along, I lost everything. THAT was a fun ride
home ...dropping the trans into low when I anticipated stopping,
left hand constantly on the parking brake lever (yea, it's a pull it
out then rotate to release type)
Just before I left for Australia this past February I blew a brake
hose on a front caliper on my wagon. Again, rears were on the loose
side and Bingo, no brakes. Luckily the car's a 4 speed and it was
fairly late at night. Got that one home, too. Fixed it when I got
back 5 weeks later.
My old Volare wagon lost caliper hoses twice with the same results and
my GTX did it to me once way back in 1971.
Now, I currently own two vehicles with single master cylinders. One's
my hemi-powered '34 and the other is the '48 Chrysler 7 passenger
sedan. Both of those cars have lost wheel cylinders and master
cylinders but on EVERY occasion there was prior warning that things
were about to fail.
Spongy pedal, rebuild the master cylinder. Low pedal, top off the
master cylinder and rebuild the leaky wheel cylinder. Drop of brake
fluid on my toe, top off the master cylinder then rebuild it when I
got home.
Normally, low brake pedal, adjsut the brakes.
I'll probably convert the '34 to a dual master cylinder but not
because it's the politically-correct thing to do, because it isn't. I
can't get rebuild kits for that old Ansen swinging pedal assembly any
more.
skip
Early hemi s.m.e.
Street rod wiring consultant
Free lance rod & custom journalist
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