by Skip Readio
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 brakes 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, adjust 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.