Brake It to the Limit – Behind the Scenes of Brake System Components
When you trace hot rodding through its early days, brake performance was often a secondary thought compared to speed and appearance. In fact, the brakes were almost more of an afterthought, a necessity of course, but not a huge priority. Just look through any vintage magazines and you see ads for intakes, cams, clutches, tires, gauges…not a lot of brake system components.
Today however, brakes are at the forefront of the performance world, especially when it comes into cars that get built to drive and, of course, in the restomod and pro-touring world. But it’s not all about 14-inch rotors and six-piston calipers, there are plenty of upgrades for the classic cruiser, including new power assist assemblies, disc upgrades, and other enhancements to improve the braking performance and safety for your favorite classic.
With all the brake systems, upgrades and kits available to today’s rodder, there is still a lot happening behind the scenes when it comes to assembling an effective brake system. Between the brake pedal and the friction material of the brake pad or caliper, there’s a lot to hash out including principles in hydraulics and physics! After talking with a few brake manufacturers, we thought we were due to walk through some of these important features, components, and considerations.
Sometimes it’s good to get back to the basics and to take a look behind the system to get a better understanding of what it takes to efficiently and consistently bring our classics to a safe stop. And this just scratches the surface!
Fluid
You can’t have a hydraulic system without fluid. Hydraulic action is the practice of using uncompressible liquid under pressure to transmit force and energy to convert it into mechanical work. When it comes to automotive brakes, that liquid is brake fluid – and there are a few different versions to consider.
Brake fluid undergoes extreme pressures and some serious heat cycles, not to mention that it must have a low freezing point and extreme boiling point. It also needs some lubrication characteristics and corrosion prevention qualities. The most common fluids are specified by the Department of Transportation as DOT3, DOT4 and DOT5.
DOT3 is the base automotive fluid but is not as capable of performing in higher performance/racing applications compared to the DOT4 blend. DOT4 has a higher boiling point and can be mixed with DOT3 fluid. DOT 5 however, is a silicone-based fluid which will not destroy paint if spilled, but it cannot be mixed with DOT 3 or 4 fluid and is not recommended for racing applications.
To confuse things a little more, there is also a DOT 5.1 which is generally used on modern ABS systems. DOT 5.1 can be mixed with DOT3 and 4 – but not DOT5! If you have any questions about your application, it’s best to discuss it with the brake manufacturer or fluid specialist.
Master Cylinder
The master cylinder is the heart of the brake system and plays a direct role in the resulting pedal effort, modulation, and the overall braking effectiveness of the system. It has a reservoir to hold the brake fluid, and it converts the mechanical effort from the brake pedal into hydraulic pressure to activate the brake calipers and/or drums.
When selecting a master cylinder, we highly advise using the recommendation of the brake system manufacturer as there’s more to selecting a master cylinder than you might think. Inside the cylinder is a piston that is pushed through a bore by a rod connected to the brake pedal assembly. The action of the piston pressurizes the system. Within the cylinder, there are small ports that direct the fluid to the proper brake circuit.
There are different bore sizes available such as 7/8-inch, 15/16-inch, or 1-inch diameters. A larger bore will create more fluid displacement but will require more pedal force, while a smaller bore will produce more pressure. Many factors come into play when selecting a master cylinder, from the calipers or drums being used, to the vehicle weight or even seat position. One of those variables is pedal ratio.
Brake Pedal and Assembly
The brake pedal assembly acts as a simple lever that forces the pushrod into the master cylinder to pressurize the fluid system. However, it’s not quite as simple as that. The length of the pedal lever and its pivot point position both have an effect on how much force is required to activate the brakes and how much pressure is applied within the master cylinder.
In most cases, the factory pedal setup will be used, so this “pedal ratio” has already been determined, though in some cases you’ll find that there are two pivot points on an OEM pedal lever; one for manual and one for power assist. Pedal ratio is the distance from the pivot point of the brake lever to the middle of the pedal push pad in relation to the distance from the pivot point to the pushrod connection. For serious performance builds or custom applications, there are many aftermarket pedal assemblies available that mount to the floor, the frame, or firewall, and have adjustments to position each pedal exactly to fit your needs.
By varying the pedal ratio, you can adjust the brake fluid pressure without changing the amount of pressure you’re applying by foot. The trade-off is that the amount of lever movement necessary will change. A very short pedal ratio (less pedal movement) may create a brake system with little ‘feel’ or modulation. Conversely, a softer, longer pedal may be slow reacting and take too much movement to effectively slow the vehicle.
Power Assist
To ease the pedal feel as the pushrod moves into the master cylinder to build pressure, there is the option of power assist. There are those drivers who enjoy the manual brake experience, but keep in mind that disc brake systems can require approximately 900-1,200psi to function properly, which can take some serious leg muscle. For street cruisers, power assist provides a comfortable and consistent brake feel and action.
Up until recently, there were two main methods to achieve power brake assist: vacuum or hydraulic. The newest power assist option is electronically aided assist. The most common practice is the use of a vacuum-operated booster; the large, round assembly positioned between the master cylinder and the firewall.
The vacuum booster uses engine vacuum and atmospheric pressure to assist the pushrod movement into the master cylinder. Typically, an engine needs to be able to create a minimum of 16-inches of vacuum and you’ll need to consider space under the hood to fit the booster. Smaller boosters may fit better, but they require a stronger vacuum signal from the engine compared to their larger diameter counterparts. If space is a premium or your engine has a rowdy cam, a hydraulic brake assist system may be considered.
A hydraulic brake assist system, commonly referred to as hydroboost, is used in conjunction with the power steering pump. This style of brake assist was originally used on vehicles with diesel engines and has found its way into the hot rod world, especially on big cammed engines that lack vacuum. This is a good option when there is limited space, or no desire, to install a big canister hanging off the firewall.
It should come to no surprise that electronic boost is a reality, since there are more electric cars on the road today, all with power assist braking. Speedway Motors offers a compact electronic system and master cylinder combination, while Classic Performance Products recently introduced their E-Booster system that is mounted behind the master cylinder. Both systems are worth checking out for high-power engines that create low vacuum, or just if you’re looking for newer technology.
Brake Pads
A brake pad is just a brake pad, right? Wrong! When you go searching for brake pads in the performance aftermarket, you’ll find that there are pads with different braking characteristics. The pad compound consists of different parts of organic materials, and even bit of metal particles or ceramic materials, that are bonded through pressure and secured with an adhesive.
There are three main categories of brake pads: organic, semi-metallic, and ceramic. Organic pads provide adequate braking for street cars but tend to wear sooner and produce a considerable amount of dust. Semi-metallic compounds are resistant to fade and will last longer but require more hydraulic force and may produce a little more noise. The other alternative is ceramic compounds, which provide strong durability combined with resistance to fade, but are a bit more expensive. Once again, it all boils down to your goal and specific application.
Valves
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epending on your setup and application, there are several accessory valves that may be required in your brake system. When it comes to hot rod applications where the master cylinder is mounted under the driver’s floor on the frame, you’ll need a residual valve on the front and rear brake lines. These valves prevent the fluid from draining back from the higher-mounted calipers or wheel cylinders and down to the low-mounted master cylinder.
There are typically two different residual valves: a 2-psi version and a 10-psi unit. For a disc brake circuit, the 2-psi valve is used and will keep the fluid from draining back while maintaining pressure in the brake line to prevent excessive pedal travel or having to pump the pedal to have full braking capabilities. For drum setups, a 10-psi valve is required.
There is also a proportioning valve that is used with disc or drum systems. A proportioning valve is typically installed on the rear brake line circuit allowing you to adjust or limit the rate of pressure increase at the rear wheels. By limiting or slowing the pressure rise, the valve prevents the rear wheels from locking up under hard braking to compensate for weight transfer.
A metering valve is yet another assembly that is typically used on vehicles with front discs/rear drums and is tasked with equalizing the pressure between the front and rear. In theory, the rear brakes should be energized before the front discs to prevent the car from dipping excessively in the front or locking up the front wheels first. The OEMs used application-specific metering or combination valves that were designed for a specific vehicle weight and size. For hot rods and custom applications, using a separate proportioning valve is the better choice.
When it comes to the brake system and components, do your homework, follow the instructions and if you have any questions, be sure to ring up the techs at: Amsoil, Baer, CPP, Johnson’s Hot Rod Shop, Speedway Motors, Wilwood
