Hydraulic lines made of tubes and hoses are used to transmit fluid under pressure between the master cylinder and each of the wheel brake units. Several valves may be used in the system. A warning light pressure switch is used on all brake systems. A combination valve is used on some front disc brake-equipped cars to improve brake balance between the front disc brakes and rear drum brakes. A stop light switch is used to signal other drivers during a stop.
Hydraulic tubes are used to direct fluid between stationary brake parts. Most hydraulic tubes used in the brake system are double wall, welded steel tubes, coated to resist rust. The tube ends are double flared or have a chamber-type flare to guard against leakage. Threaded fittings are used to connect the tubes to brake parts.
The wheel brake units move up and down with the suspension. The master cylinder and steel brake tubes are mounted to the stationary body and frame. Flexible hoses are used to connect stationary brake components with moving components. Hoses must be able to withstand high fluid pressures without expansion and must be free to flex during spring deflection and wheel turns without damaging the hose. Hoses come in different sizes and lengths, with a variety of end fittings to accommodate different vehicle requirements.
The stop light switch is a spring-loaded electrical switch installed in the vehicle's stop light circuit. In some installations, the switch is operated by hydraulic pressure. In others, the switch is mechanically operated through contact with the brake pedal. With the brakes released, the circuit through the switch is open. When brakes are applied, the switch closes to complete the circuit to the stop lights.
Most late-model cars use a combination switch that contains a warning light switch, a metering valve, and a proportioning valve. The warning light switch is designed to light a brake warning lamp on the instrument panel if there is a hydraulic failure in either side of the split system. The switch works by sensing a pressure difference between two hydraulic circuits.
The switch body has connections for the hydraulic lines from the master cylinder. Outlet connections go to the two separate systems. This can be the front and rear wheels. On the diagonal style, a left front and right rear is paired together. The right front and left rear are separate systems.
A small switch piston is positioned between the two hydraulic systems. A spring positions the switch piston in the center of the switch. When the pressure is equal in both the front and rear hydraulic systems, the switch piston remains centered and does not contact the switch terminal in the bore of the switch.
If pressure fails in one of the hydraulic systems, hydraulic pressure moves the piston toward the inoperative side. The piston moves off-center to push up a switch pin. The switch pin activates the contacts inside the electrical terminal. This provides a ground for the warning lamp circuit and lights the warning lamp.
Most cars that use front disc brakes and rear drum brakes use a metering and proportioning valve in the system to achieve balanced braking between the front and rear wheels. The purpose of the metering valve is to improve braking balance, particularly during light brake application.
The metering valve keeps the front discs from operating until the rear drums have started to work. The valve is needed because disc brakes are fast acting; drum brakes have spring tensions and play in linkages to overcome first. The valve is located in the line to the front brakes. It works on fluid pressure and is normally closed. When the brake pedal is depressed, the fluid flows first to the rear brakes. As they begin to take hold, the system pressure builds up enough to open the metering valve, admitting pressure to the front brakes. Once open, the valve has no effect. The effect of the metering valve is felt during the beginning stages of all brake applications and throughout the duration of light brake applications. The proportioning valve is installed in the hydraulic circuit going to the rear drum brakes. Its function is to maintain the correct proportion between line pressures to the front and rear brakes and, therefore, provide a balanced vehicle braking system.
The proportioning valve reduces the hydraulic pressure at the rear drum brakes when high pressure is required at the front disc brakes. The valve helps to prevent premature rear wheel lock-up and skidding during heavy brake applications and provides better braking balance.
The Modern Hydraulic System
When you think about it, it's pretty hard to believe that a teeny tiny column of fluid can transmit enough force to stop a ton or two of hurtling automobile, but that's the magic of hydraulics. Early cars used mechanical apply set-ups -- cams, cables, and levers. No matter how clever the design, however, they were almost impossible to equalize perfectly, and required constant adjustment, so the idea of using hydraulics to do the job intrigued engineers from about 1897 onward. But it took many years to develop reasonably dependable systems, so the first domestic car with fluid pressure-actuated brakes was the '21 Dusenberg, and Chrysler followed in '24.
With refinements, hydraulic systems essentially the same as those originals got us through four decades. But in the mid-60's the changes and complications started. First there were discs and dual circuits with metering, proportioning, and a warning light, then came combo valves, diagonally-split systems, low-drag calipers, Quick Take-Up/step-bore masters, load-sensitive proportioning, etc. (ABS is another whole subject I won't tackle here except for maintenance).
A firm grasp of the modern hydraulic system and the service procedures it requires is about as important to anybody doing brake work as remembering to breathe. Unfortunately, I've found that lots of you out there still have some fuzzy areas in that essential understanding and also harbor a few misconceptions and prejudices.
A case in point is the dual, split, or tandem master cylinder, which has been used on every car sold in this country since 1967 (although Cadillac had it as far back as '62). Plenty of people still aren't comfortably familiar with its construction and operation.
A typical modern specimen will be of the composite variety (in other words, aluminum with a plastic reservoir), but iron one-piece units are still around in abundance. Two pistons ride in the bore, the rear piston being the primary, and the front the secondary.
Each piston has a primary cup at its front and a secondary at its rear, so you'll be hearing such combinations as primary piston secondary seal, secondary piston secondary seal, etc. The primary seals are the most important because they trap the fluid that's about to be squeezed into the lines. The primary piston's secondary seal keeps fluid from escaping out of the back of the cylinder, and the secondary piston's secondary seal acts as a barrier to make two essentially separate cylinders out of one. In normal braking, the pushrod from the pedal or booster forces the primary piston forward. No pressure is created until the primary seal covers the compensating or vent port from the reservoir, but once it does fluid is trapped in the chamber between the pistons and becomes, for all intents and purposes, a solid column. Pressure is routed from this chamber to two wheels. A combination of the trapped fluid and the primary piston coil spring bears on the secondary piston, moving it forward also and creating pressure in the chamber ahead of the secondary piston's primary seal, to which the line to the other two wheels is attached.
When the pedal is released, a partial vacuum occurs in both pressure chambers because the fluid is too lazy to return from the lines fast enough. So, in order to re-arm the brakes instantaneously, the primary seals are designed to allow fluid to flow one way (forward) from behind each seal into the pressure chambers.
The replenishing ports allow fluid to move freely between the chambers behind both pistons' primary cups and the reservoir according to demand and expansion and contraction from temperature changes.
If a hose lets go or a saboteur has sawed through one of the brake lines so there's a catastrophic loss of fluid in half the system, the other half will still provide a means of decelerating the vehicle, albeit with a lower pedal and reduced stopping power. Both pistons have extensions which project out in front of their primary seals. A failure in the circuit that's connected to the primary piston's pressure chamber will allow the piston to move forward enough so the extension will bear on the secondary piston, push it ahead, and generate pressure in the other circuit. If, on the other hand, the circuit that gets its juice from the secondary chamber blows, the extension on the secondary piston will bottom out on the front of the cylinder and the fluid trapped between the pistons will operate the alternate set of brakes.
The residual pressure valve was once common in outlet ports that go to drum brakes. It maintained 5-20 psi in the lines to keep the wheel cylinder cups in constant contact with their bores. Most cylinders today have those little metal expanders behind the seals that make this unnecessary. If somebody were to install a check valve in a disc brake circuit, he'd create a constant dragging condition because discs are designed to work with very little clearance.
Since the GM X-Car appeared in 1980, there's been another complication: the Quick Take-Up/step-bore master cylinder. The engineers were trying everything possible to get the last tenth of an mpg, so the rolling resistance that zero-clearance discs caused was quite enough to warrant the adoption of low-drag calipers. Piston seal grooves were machined at an angle so the seals retracted the pads a sufficient distance to eliminate this parasitic loss, but that meant a master that displaced a large volume of fluid quickly was required.
The design arrived at uses a stepped bore and a primary piston with a small front and a larger rear diameter. During the first part of the stroke, the large part of the piston naturally displaces more fluid than the small part, and this extra volume goes around the lip of the small seal into the chamber between the primary and secondary pistons, moving the secondary ahead more than the distance the pushrod has traveled. That way, extra fluid is displaced into both circuits. A logically-named Quick Take-Up valve, which is connected to the rear high-volume chamber, vents excess fluid up into the reservoir once a certain psi is achieved, and also acts as the refill passage for the large chamber.
Other manufacturers have followed suit, as with the Tokico master you'll find on some FWD Fords, which features what's called a "Fluid Control Valve" to give essentially the same action as the GM unit.
But a master cylinder alone does not an integrated brake system make. Means of fine-tuning the pressure for the situation and warning the driver of a partial failure are equally important. All kinds of individual and combination valves are used to perform the metering, proportioning, and warning light activation functions, and I'll consider these jobs one at a time.
Disc brakes operate with very little clearance between the pads and the rotor, so the instant the caliper receives pressure, the drag on the wheel begins. But drums are different. There's considerable space to be taken up before the shoes go to work. If a disc/drum combination were connected directly to the same master, the discs would end up doing far more than their share. The metering or hold-off valve is what divides the load fairly. It stops the flow of fluid to the calipers until pressure reaches 75-125 psi or so, then it opens. This gives the drums a chance to catch up, so both types of brakes start applying at the same time. If you use a pressure bleeder during service, the metering valve will have to be deactivated, which is usually done by pulling on or depressing a pin.
Since drum brakes are self-energizing and commonly duo-servo whereas discs work entirely by means of hydraulic force, drums tend to lock up in hard stops. Weight transfer adds to the problem of rear over-braking, so even vehicles with discs all around need some means of keeping the posterior decelerators within bounds, and the proportioning valve was invented to do the job.
This device limits the flow to the rear brakes after a certain pressure has been reached, and this "split point" can be anywhere from 200 to 500 psi. Above that, force in the rear lines is allowed to rise at only a portion of the maximum available. The valve does absolutely nothing in normal, low-pressure stops. On front/rear split systems, if the front circuit fails, the valve is bypassed to allow full hydraulic power to reach the rears.
A further refinement is the load-sensing proportioning valve, which you'll find on various pickups, vans, utility vehicles, and even some cars (the first one I ever saw was on a '71 Fiat). The idea here is to use the distance between the body and the axle to adjust rear stopping power to match the weight on the posterior wheels and prevent lockup. Linkage connected to a lever on the proportioning valve varies the pressure available. You'll have to bleed this set-up with the vehicle's weight on its wheels because the valve will shut off the flow if the axle is hanging. Typically, these valves have adjustable linkage, but be certain to follow the specific service recommendations before fooling with them. One mm of adjustment can change pressure by 60 psi.
The extra safety the dual brake system provides carries a subtle danger with it: If one half springs a leak and the driver isn't sensitive enough to notice that he's pushing the pedal harder and farther than normal, he might ride around indefinitely with severely inadequate brakes. So, a dash light is provided that winks on when one circuit is in trouble, and it's activated by the pressure differential switch. Basically, this is a piston that remains centered in its cylinder as long as there's equal psi in both circuits. If one side blows, the pressure of the other pushes the piston toward the open side, and this movement closes the switch to the warning light.
What's the best way to divide a dual system? Well, the original domestic approach was to put both fronts on one circuit and both rears on the other. That works fairly well should the posterior brakes blow. But if the fronts go out, you've got vastly reduced stopping power and the tendency to skid if you lock the wheels. With FWD the situation became untenable because the rear wheels are so lightly loaded.
Enter dual-diagonal wherein one front and one rear on opposite sides share a circuit. This added the need for some additional plumbing (two proportioning valves, for instance), but gave more reasonable emergency deceleration ability in return. Bleeding sequence changed from the traditional RR, LR, RF, LF to RR, LF, LR, RF.
The ultimate system for safety is what you'll find on non-ABS Volvos: Both fronts and one rear will operate even if one circuit fails. This is accomplished by using two- or four-piston calipers with one side of each plus one rear brake connected to half the system. One pad of each disc brake will always be operational.
The metal brake lines that route pressure to the wheels haven't escaped change. They're still made of double-wall steel, but a different type of fitting is taking over. Called the ISO (International Standards Organization) flare, it's not compatible with traditional double-flare/tube seat connections. Because the shoulder of the nut bottoms in the fitting, sealing pressure is uniform and over-tightening ceases to be a problem. Also, only one simple die is required to form the flare.
Then there's brake fluid, the stuff that makes everything happen. Modern DOT 3 and 4 glycol are both pretty good because they have high boiling points and the ability to hold lots of moisture. But that affinity for water made another type seem attractive: silicone, rated DOT 5 and color-coded purple. It doesn't absorb H2O, so was expected to practically eliminate corrosion, has a 500 degree F. boiling point, and won't dissolve paint the way ordinary glycol does.
Why hasn't everybody gone to silicone? Because there are drawbacks and unanswered questions. First, it's way more expensive than DOT 3 or 4, but a few dollars wouldn't really make much difference in the average price of a car today. It's the unresolved performance characteristics that have kept the carmakers from filling hydraulic systems with DOT 5 at the factory. Seal life is one problem. An aftermarket brake engineer I know told us he has trouble getting cylinder cups to make it through the SAE longevity test with silicone -- they get hard and wear out, he says. And, if any moisture should find its way into the system, it tends to collect in slugs. When elevated temperatures are encountered, especially at high altitudes, these can turn to bubbles resulting in a loss of stopping power.
The auto makers just don't consider silicone's potential advantages worth the risk. After all, a typical brand-name super heavy duty DOT 3 has a dry boiling point of 485 deg. F., which is only 15 deg. less than silicone (wet, this falls to 310 deg., but there's no excuse for running around with watered-down fluid, as I'll explain).
Now for some service tips that'll help you avoid problems. The first thing I'd like to mention is that you should always open the bleeder before bottoming out those caliper pistons. Otherwise, you'll back flush nasty sediment up into the works. This is bad enough with a regular master, but with all the tiny passages in an ABS it can cause big trouble. Just do it.
Then there's flushing and refilling the system with fresh fluid. In the past, some brake experts said it wasn't worth the effort because you can't get all the old stuff out unless you disassemble the calipers and cylinders. True, you won't be able to eliminate every drop of the contaminated liquid, but you can get most of it, and that will effectively reduce the amount of moisture in the circuits.
This has always been important for corrosion prevention, but now the high operating temperatures encountered with semi-mets and FWD make maintaining a high boiling point critical to safety even for the average motorist. Besides water, there's sediment, which is a combination of rust and the ashy residue of burned glycol. Expensive and intricate ABS hardware is further justification for this maintenance. Many of these systems are vented to the atmosphere, and there's also contamination from under-hood vapors in some layouts. Fluid changes are cheap insurance against big-bucks repairs.
Depending on the authority, recommended intervals range from one to three years.
Whenever you've got a car up in the air, take a careful look at the metal lines and rubber hoses. In my own shop, I see total circuit failures due to corroded lines or blown hoses frequently enough to be very conscientious about this. If a line runs up over the chassis so that it's hard to see, use a mirror and your sense of touch.
Replace lines if the rust has reached the scaly stage. When installing replacements, follow the original routing as closely as possible. I often hear of fade or low pedal problems because one of these lines has been mounted too close to an exhaust pipe, or has a hump or loop in it that traps air.
Those flexible hoses are so well made they often survive for the life of the car, but why push your luck? I almost took an icy dip in Lake Michigan when one blew on my pre-dual master '66 Ford, so I consider their replacement valuable insurance. You could use some carmakers' service literature to support the idea of new hoses as regular maintenance -- the Ford Fiesta manual, for example, says hoses should be retired after 36K miles. A related item is the high pressure ABS hose between the pump and the accumulator, which some car makers want you to replace at 60,000 miles.
Air expulsion definitely deserves some space because there's more to it than just observing the proper sequence. Bench bleeding master cylinders, for instance. Neglecting this is the number-one reason for spongy pedal complaints, and some re-manufacturers include fittings, tubes, and instructions in the box in hopes of reducing the number of unnecessary returns.
You can do this job by just holding your fingers over the outlet tubes to keep air from being drawn in on the return stroke, but that's pretty messy, so I'll give you the procedure using tubes. Clamp one of the master's mounting ears in a vice so the unit is as level as possible. Position the tube tips well below the level of fluid in the reservoir, then use a rod or drift to stroke the piston SLOWLY. Especially on Quick Take-Ups and their equivalents, wait at least 15 seconds between strokes to allow the low pressure chamber to release all its bubbles and fill completely. Keep stroking until there's no more evidence of air at the tube tips and ports.
Should you get a car with a replacement cylinder that some other service personage didn't bench bleed, you might be able to do it with the master in place providing you can jack the rear of the vehicle high enough to get the cylinder level. Surge bleeding -- you know, where you pound the pedal violently a bunch of times to get the bubbles mixed up with the fluid, then crack the line -- is frowned upon by experts who don't think aeration is ever a good idea.
By the way, one major brake outfit (EIS) has done us a favor by providing aftermarket step-bore master cylinders with a bleeder that facilitates getting out all that trapped air. Located in the low-pressure side of the cylinder below the Quick Take-Up valve, it gives those bubbles a convenient exit path. This little feature can reduce bench bleeding time to maybe three minutes, and practically eliminates low-pedal comebacks.
When it comes to the bleeders at the wheels, I know most of you just open them and let the fluid squirt. But besides making slippery puddles on the floor, it can shoot farther than you might expect, perhaps ruining the paint on a nearby car. I use a transparent tube and bottle set-up that hangs by a magnet because it's neat and it lets me see what I'm getting out. Also, it eliminates the need for a helper if I'm not using a pressure bleeder.
Then there are the bleeder screws themselves. You haven't worked on cars very long if you haven't encountered a frozen one that breaks off before opening. My rather drastic and primitive method of clamping Vise-Grips to the screw and shaking it while I heat the cylinder or caliper around the port with a propane torch often works, but there's a better answer. Whenever you get a vehicle in that hasn't yet developed the problem, unscrew the bleeders and coat their threads with just a touch of anti-seize. Ditto when you replace a caliper or cylinder. If you get that conveyance back later for brake work, you'll be glad you took the time.
In an ideal world, every caliper would be overhauled during a reline to insure against piston seizure and seal failure. And that's what most authorities recommend. But when was the last time you encountered a leaky caliper? They last and last in most cases. So, if you're under time or cost pressure, you could just push those pistons back (bleeder open, of course). One caveat: If you feel any roughness or binding as you force a piston home, you'd better get inside.
What about wheel cylinders? The frequency of leaks here is so high most of the good techs overhaul or replace them in every case.