Brakes 
Canberra Hot Rod Association (CHRA)
Disc / Drum Set-Up
Disc / Disc Set-Up
Correct Valve Installations
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VALVING
In order for your braking system to function properly the system must be properly balanced. This is accomplished through the use of the correct valves. Below is a description of what types of valving may be used on your system.
HOLD-OFF / METERING VALVE
are used in the front (disc) system of a disc/drum brake system. The metering or hold off valve is used in the brake system to better balance the front to rear brakes. They provide a "hold off" function to allow the rear (drum) brakes to actuate first. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. This function is very important in making the system function in the correct sequence in a rear wheel drive car. The rear brakes are always actuated first. At this point the valve opens to allow full pressure to build at the front brakes. This function is built into most factory type disc/drum combination valves. Make sure you have a metering/hold-off valve in the system either as a stand alone valve or as part of a factory valve.
RESIDUAL VALVE
There are two different residual valves. A ten pound residual valve will maintain a line pressure to the rear to keep the drum brake shoes out close to the drums giving a higher firmer pedal. Without a ten pound residual pressure to the rear you will experience a spongy pedal. A two pound residual valve is required whenever the master cylinder is lower than the calipers to prevent backflow of fluid from the calipers to the master.
Residual Pressure Valves are used in a both front and rear brake system as follows:
- 2 PSI Valves - These valves are used in a disc
brake system only and are required when the master cylinder is at, or
below, the height of the calipers. It's purpose is to act as an
anti-siphon valve preventing the brake fluid from siphoning back into
the master cylinder when the brake pedal is released. Even if the master
cylinder is even or slightly above the calipers, put one in anyway. If
you don't and you park on a hill, fluid will siphon! These valves are
cheap insurance - put them in! NOTE: You will know if you need one of
these valves if you had to pump the pedal twice to get a good pedal. See
illustration for more.
- 10 PSI Valves - These valves are used in a drum
brake system to prevent air from being ingested into the hydraulic
system when you release the brake pedal. Typical wheel cylinder seals
only seal when there is pressure behind them. Rapid release of the brake
pedal creates a vacuum in the system which causes the seals to relax and
air is ingested into the wheel cylinders. Maintaining 10 PSI in the
system at all times prevents this. Some disc/drum master cylinders have
10 PSI residual pressure valves installed internally, some don't. If
you're not sure, call us and we can tell you how to check. Also, some
new style wheel cylinders have cup expanders which negate the need for
the residual pressure valve. Either way, if you are not sure whether you
have one or not, put one in. They are not cumulative and it won't hurt
anything if you have two. Don't worry about brake drag, it takes roughly
75 PSI to overcome the return springs.
COMBINATION VALVE
A combination valve incorporates metering, proportioning and 10 pound residual into one valve. These are available for disc/drum or drum/drum systems.
PROPORTIONING VALVE
Go in the rear brake system (disc or drum) and provide for control of the rate of pressure rise to the rear brakes -- just the rate at which it builds up. Sooner or later the rear brakes see full master cylinder discharge pressure. The proportioning valve modulates the pressure to the rear brakes. The modulation is necessary to minimize rear wheel lock up found in heavy braking and to compensate for the differences in braking conditions in front disc / rear drum systems. The purpose of this rate of pressure rise control is to compensate for the reduction of weight on the rear wheels due to forward weight transfer during braking. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lock up. In short, it eases the application of the rear brakes to help prevent rear wheel lockup. Factory combination valves have these built in -- make sure you know what you're getting, too little rate of rise is as bad as too much. You may not need one of these valves depending on the compatibility of your vehicle's brake system components.
ADJUSTABLE PROPORTIONING VALVE
Allow for fine tuning of the rate of pressure rise to the rear brakes if you have a lock up problem. If you have a lockup problem, experiment with the setting of the valve to eliminate lock up for all but all out panic stops. Good luck! ADJUSTABLE PROP VALVE The adjustable proportioning valve is used when you have a special rear condition that requires higher or lower pressure than a normal condition. You sould always use a metering valve to the front when using the adjustable
Frequently Asked Question
How does a brake booster work? A power booster uses atmospheric pressure to help assist the pushing of the master cylinder piston. Basically, a power booster is divided into two chambers internally with a rubber diaphragm seperating each. With no force applied to the booster there will be vacuum on both sides of the diaphragm. This vacuum is supplied by the engine either from the intake manifold or the back of the carburetor.
When you press on the pedal, the rod opens a poppit valve in the booster that allows atmospheric pressure on the firewall side of the diaphragm. This causes the diaphragm to push on the master cylinder piston thru a plastic hub, providing power assist to the master cylinder.
Are Dual Master Cylinders really "Safe"?
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) a car 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-adjusting brake maneuvers (better take a minute and explain what 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 rotation 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 properly 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 front-most 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 (the 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 appreciable 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 disc fluid isn't.
So, HERE'S WHERE THE FALSE REASSURANCE 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 the 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 down 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.
Hot Rod Auto Electric Consultant -More on dual master
cylinders from '97
A couple of things I've noticed with regards to dual master cylinder
retrofits.
Just did a wiring job on a '32 roadster that had been
retrofitted with a dual master cylinder to replace the old fruit jar. The
one the nationally renowned street rod supplier shipped ("we sell hundreds
of these and nobody's ever complained") was a disc/drum master cylinder on
a drum/drum car.
What's the big deal? Well, there was no 10 psi residual valve in the
front brake circuit.
The front brakes do most of the work and you don't
want air in the wheel cylinders. When you release the pedal, the return
spring pushes the piston back in the master cylinder. This lowers the
pressure in the system to a point where air is ingested through the wheel
cylinder packings (don't forget, they're shaped to hold fluid in, not keep
air out.)
Every time you step on the brake and take your foot off again, the piston in the master cylinder comes back faster than the brake shoe return springs can force the fluid back into the reservoir.
If you're building a nostalgia rod with early Ford brakes or finned
aluminum Buick drums, get the right master cylinder up front and you won't
have to add a 10 psi residual valve later on.
Another is the use of a
fruit jar on an all disc setup. The car needs a couple of people behind it
to move it around the garage.
Take out the residual valve on the end of the master cylinder piston if you're going to use a fruit jar (or ANY single master cylinder) on a disc brake application.
If you mount it under the floor and it's "looking" at disc brakes, put a 2 psi residual valve in the line to prevent the brake fluid from gravity-feeding back into the master cylinder. That'll give you that hard pedal first time around instead of after the first pump. It could mean the difference between a pristine '34 grille and one with the imprint of someone's Reese hitch.