How, What, and Why
We're all well aware of how important the braking system in our vehicles is. And, we all know what could happen if the brakes fail--the result could be down right disastrous. Unfortunately, we tend to pay much more attention to making our vehicles go fast than we do to stopping fast. For this month's issue we decided to highlight braking systems, as you can probably tell by now. Here I'm going to attempt to layout a bunch of brake basics for you. Who knows, I may have found a tidbit that you were not aware of. For example, did you know that every time you mash that pedal, the hydraulic system exerts as much as 1,000 pounds of hydraulic pressure to each of the four brakes? We all might learn something here after all.
Brakes are essentially energy-conversion devices, which alter the kinetic energy (momentum) of your vehicle into thermal energy (heat). It's this factor that I believe led to the development and popularity of disc brakes--but, I'm getting ahead of myself here. Let's get to some real basic info and then work our way up to specifics.
In the brake systems we're familiar with, the master cylinder is usually separately chambered, (one fluid reservoir for the front and one for the rear), and power-assisted to activate both the front and rear brakes. This way, if either fails the other can provide adequate braking power. Such safety systems within the braking system make modern brakes very complex, but much safer than earlier braking systems found on our beloved classic Chevys. Single chamber masters and four-wheel drum brakes aren't going to cut it these days, especially not in performance vehicles. High-performance disc brakes are the way to go. They were originally developed for racing, but are now used on most cars and a disc/drum or disc/disc conversion should be a near-top priority on your classic vehicle upgrade list.
In almost all braking systems, the brake pedal is connected to a "master cylinder" by a pushrod. The master cylinder is connected to small slave cylinders at each wheel by steel brake lines and flexible rubber hoses. The entire hydraulic system is filled with brake fluid, which is forced through the system by pressing on the brake pedal, causing the movement of the master cylinder pistons. The front disc brakes use friction "pads" which are mounted in "calipers." The pads are forced against machined surfaces of a rotating disc called the "rotor." The rear brakes are usually of the "drum" type. In these, the internal expanding brake "shoes" are forced against the inside-machined surface of a rotating drum.
In recent years, brakes have dramatically changed in design. Disc brakes, due to their lighter weight, superior cooling, and better performance are replacing drum types on the rear wheels, as well. As I stated earlier, instead of shoes, which press outward against the inside of a drum, a disc attached to the axle is gripped from either side by friction pads attached to the calipers, just like the ones up front. The greatest advantage of disc brakes all around is that they're essentially "fade" free; that is, repeated application doesn't result in excessively high temperatures developing (as in the case of drums), lowering the stopping power of the brake. Commonplace on newer cars are "anti-lock" brake systems, (ABS) which pulse the hydraulic pressure, preventing the wheels from locking up when the brakes are applied in a panic stop.
Power brake units used on passenger cars are of four general types: vacuum suspended; air suspended; hydraulic booster, and electro-hydraulic booster. Most power brakes use vacuum-suspended units, which contain a large vacuum-powered booster device to provide the added thrust to the typical power brake. Pressure on the brake pedal pushes a rod connected to the pistons of the two master cylinders forward. The pistons begin forcing fluid into the front and rear brake lines. At the same time, the brake-pedal pushrod positions the vacuum-control valve so that it closes the vacuum port and seals off the forward half of the booster unit. The engine vacuum line then draws off the air, creating a low-pressure vacuum chamber. Atmospheric pressure in the control chamber then pushes against the diaphragm, dividing the two chambers, the pressure on the diaphragm, which is locked to the pushrod, forces it forward, supplying even more pressure on the pistons.
CALIPERS AND ROTORS
At the heart of the modern brake system are the caliper and disc rotor (which rotates with the wheel). When the brakes are applied, the caliper tries to "grab" and stop the rotor. The efficiency of this system is very good, and depends on a few basic factors:
The ability to apply enough clamping force to overcome the rotating rotor. The ability of the pads to maintain adequate friction properties even with the heat generated by continuous application of pressure from the caliper. The ability of the rotor to absorb and dissipate heat. AND the necessary traction from the tires to allow the brake system to be effective. Also, there are different types of calipers. A "fixed" caliper is rigidly secured to the axle assembly and has at least two opposing pistons that force the pads against the disc. A sliding or "floating" caliper has pistons on only one side of the disc. Therefore, when the caliper acts, it must slide or float in order to bring the pad on the opposite side in contact with the disc. Nearly all of the original equipment calipers are of the floating type. In a system with fixed calipers, not only is the mounting much more rigid, but the stiffness of the caliper itself is greatly increased; this manifests itself in enhanced braking performance, pedal feel, and pad wear.
Also, fixed calipers do not suffer the drag (friction) from moving parts common in sliding calipers and are therefore more efficient. These calipers have a much quicker reaction time, which translates to shorter stopping distances as measured from the driver's reaction point.
If we increase the area of the caliper pistons by increasing their diameter, the caliper will apply more clamping force to the pads and rotor, further increasing the braking capacity of the vehicle. Because the hydraulic pressure is constant throughout the system when the brake is applied, the more caliper pistons we add, the more pressure is exerted by the caliper; more pressure or clamping power is more effective in preventing rotation of the rotor.
The distance from the center of the rotor to its edge (radius) can be thought of as a lever, and the caliper in effect pulls on the lever to slow the vehicle; the bigger the rotor, the longer the lever--the longer the lever the more effective the caliper. It is easy to understand then that larger rotors increase braking capacity. If the rotor is also well ventilated, its ability to dissipate heat is improved, allowing the caliper pads to operate at a lower temperature and tolerate the torture to which they are subjected. When holes are drilled through the rotor and slots cut into the surfaces of the rotor, it provides channels through which the expanding gases can escape, increasing the brake efficiency a step further over a solid rotor. However, these rotor-surface treatments do not automatically guarantee a shorter stopping distance, but they usually can. As mentioned, cross-drilling and slotting is useful when the pads become sufficiently hot to emit gases. Without an escape route, the gases are trapped between the pad and rotor and actually reduce the pad pressure to the rotor (brake fade).
Different pad compounds offer different friction coefficients and the ability to work well at certain temperature ranges. Most often, a compromise is reached between the friction coefficient and the ability of the compound to operate at the expected temperatures. The higher the friction coefficient of the pad compound, the more grip it has on the rotor, making the brakes more effective.
In the event that the brake system is used to full capacity, even with all of the improvements and the rotor pads are overheated, they will give off gases as a result of the bonding agents in the pad lining that are beginning to burn. The expanding gases from the pad ("out gasing") form a cushion between the pad and the rotor, rapidly decreasing braking effectiveness. This condition is referred to as "brake fade."
Probably the least expensive, most effective upgrade one can perform is a brake pad upgrade. There are as many pad compounds as manufacturers of pads, and each has its particular niche. Brake pads are available in three basic categories: Street, Street / Race combination, and Race Only. Street pads have to be able to work well at low to moderate temperatures because street vehicles are driven cold and under normal circumstances don't generate high temperatures. Usually in this category, the pads work well while cold and their effectiveness decreases as their temperatures increase until the breakdown of the compound bonding agents cause brake fade. Repeated hard brake applications, as in most types of racing, will quickly overheat these pads and render them useless. Combination pads usually incorporate some degree of compromise to incorporate this flexibility of use. For the street they have to work well enough at low temperatures to be safe and must also be competitively functional at moderately high temperatures under racing conditions. Race Only pads do not work cold--period. Do not use Race Only pads on a street vehicle with the idea that "If they are for racing they must be great pads." You will be in for a huge surprise the first time you go to stop.
All the brake improvements in the world are not going to help if there is no traction available between the wheel and road. Ultimately, your tires will determine how well your car stops. A brake system of proven efficiency will be most effective with the smallest possible diameter tire. In the same way that the larger rotor offers the caliper a longer lever, the smallest diameter tire offers the a smaller lever. While different (smaller) wheel and tire diameters are frequently an option for racers, most street vehicles are limited to near-standard tire diameters for a variety of reasons. Fortunately, the trend in recent years toward larger diameter rims and low profile tires has opened up many options in the area of big brakes. In many cases, increasing rotor diameter will require an increase in rim diameter to make room for the rotor. Fortunately, low profile tires keep the outside diameter the same or nearly the same as the original equipment. Increasing the rim diameter, therefore, usually has no ill effect on braking. Conversely, modern low profile tires are typically of a higher traction rating (or at least high traction ratings are available) which will aid braking.
Frequently Asked Questions
Q. How can I tell if my master cylinder is bad?
A. When your master cylinder goes bad you will have a very spongy pedal, and if you keep pressure on the brake pedal it will slowly sink to the floor.
Q. What's the advantage of replacing my original single piston master cylinder with a dual piston?
A. Safety. If you brake a line with a dual piston master you will still have half the system available as a backup. With a single piston master you will lose all your brakes.
Q. Should I purchase a new or rebuilt master?
A. Given the opportunity choose the new over the rebuilt. The additional cost of a new master is easily justified by the fact that new cylinders have a much lower failure rate than a rebuilt one.
Q. I purchased a replacement master cylinder and it doesn't look like the original. Can I use it?
A. Probably. Many aftermarket master cylinders have a different casting but internally they are the same. Avoid cheap imports!
Q. How does a disc brake master cylinder differ from a drum brake master?
A. A drum brake master will differ from a disc brake master in two ways: the amount of fluid that a drum brake master moves is less than that moved by a disc master and drum masters have 10-pound residual valves to the drums. If you use a drum master for disc brakes you would move an insufficient volume of fluid and the disc brakes would drag because of the residual valves.
Q. What is the difference between a power brake master and a manual brake master?
A. As a general rule, a power brake master will have a larger bore diameter than a manual brake master. Also, a manual master will have a deep piston hole to accept the manual brake push rod while a power brake master will have a shallow hole.
Q. What bore size and pedal ratio is needed for a manual brake master cylinder?
A. For manual brakes you should always use a master cylinder with a bore size of 1 inch or smaller with a pedal ratio of 6: 1.
Q. I have manual brakes with an extremely hard pedal. Why?
A. Check the bore size of the master, it's probably larger than 1 inch. Also check that your wheel cylinders are not frozen.
Q. Can I use my disc/drum master for four-wheel disc brakes?
A. No. The addition of rear disc brakes requires a true four-wheel disc brake master cylinder which will supply more fluid pressure and volume to the rear calipers.
Q. Why is one chamber in a disc/drum master larger than the other?
A. The master cylinder chamber that feeds the front disc brakes will have its fluid level drop faster than the drum brake chamber because the front pads wear faster, allowing the caliper pistons to extend outward.
Q. Can I use my manual brake master cylinder on a power booster?
A. Yes. However you must be sure that the booster pin length matches the hole depth of the master cylinder pistons.
Q. Can I use my power brake master cylinder for manual brakes?
A. No. The bore size of the power master will probably be too large and the piston hole depth will be too shallow.
Q. How do I increase my pedal ratio when I convert from power to manual brakes?
A. Attach the manual brake push rod 1 inch higher on the pedal than the power brake attachment point.
Q. Do I need to bench bleed my master cylinder before installing it?
A. Yes, always bench bleed before installation.