How to Choose a Master Cylinder and Booster


How to Choose a Master Cylinder and Booster


While brake system components might not have the sex appeal of coilovers or a turbocharger, they play a crucial performance role in virtually any car. All the horsepower in the world isn’t worth much if you can’t reliably bring the car back down from speed, so having consistent, predictable stopping power is critical if you want to extract every last bit of capability from a given setup. And as Mark Fowler of Baer Brakes points out, master cylinder and brake booster selection can have a profound effect on the end result.

“If you have an incorrectly sized master cylinder for the brake system you’re using, it can result in either a soft and spongy brake pedal, or a pedal that’s as hard as a rock and requires you to basically stand on it with both feet to create enough line pressure to get the vehicle to stop. Brake booster selection will also affect pedal feel and overall response, but with boosters, the option that makes the most sense for a given application often comes down to the packaging constraints.”

Determining which master cylinder and brake booster are right for a particular project involves the consideration of several important factors that are often application specific. Here Fowler walks us through the variables that should be top-of-mind when selecting these parts.

Choosing the right master cylinder and brake booster begins with an honest assessment of how you plan to use your vehicle. A mild street cruiser will have considerably different requirements than a pro-touring car set up for track days and autocross, or a dual-purpose drag car.

Choosing a Master Cylinder

The master cylinder’s role in a brake system is to provide line pressure to the calipers. That pressure is then multiplied to create the clamping force that’s applied to the rotors to slow the rotational speed of the vehicle’s wheels. From a functional standpoint, selecting the correct master cylinder for a project ultimately boils down to bore sizing.

“The first question that we always ask folks here is whether they’re using manual brakes or power-assisted brakes,” Fowler says. “If someone is using a power-assisted setup, they’re going to run a larger master cylinder than they would with a manual brake system. From there, we identify what calipers they’re using, which will tell us the piston area of the caliper. That also affects the sizing of the master cylinder.”

Baer billet aluminum Remaster master cylinders are available with right or left port exits, to accommodate the needs of different vehicle applications.

But that piston area isn’t necessarily determined by the number of pistons in a caliper. “It’s determined by the size of the piston, or pistons. It’s possible to have just as much – or even more – piston area in a single-piston caliper as you’d find in a multi-piston caliper. The main advantage of multi-piston calipers is improved control over the brake pad. Instead of just pushing in the center of the pad, a multi-piston setup allows it to push from several different areas on the backing plate to apply pressure to the pad more evenly.”

A larger piston area requires a higher volume of fluid, which in turn dictates the use of a larger master cylinder. “With a smaller piston area, you’re going to reduce the size of the master cylinder not only because the caliper doesn’t need that volume of fluid, but also because the smaller the master cylinder’s bore diameter is, the more line pressure you get. With everything else being equal, a one-inch master cylinder is going to create less clamping force than a 15/16ths master cylinder.”

The piston area of the brake caliper is a key factor in deciding what master cylinder will be most effective. Contrary to what many people think, more pistons in a caliper doesn't necessarily mean more piston area. "The main advantage of multi-piston calipers is improved control over the brake pad," says Mark Fowler of Baer Brakes. "Instead of just pushing in the center of the pad, a multi-piston setup allows it to push from several different areas on the backing plate to apply pressure to the pad more evenly."

Baer’s Remaster master cylinders are constructed from billet aluminum, but the company also offers OE-style master cylinders made from cast iron. While that design element is largely an aesthetic choice, there are other options available that are more application specific.

“Aside from the bore sizing options, we also offer them with left or right port exits,” Fowler explains. “And that means the lines leading out of the master cylinder are either going to exit toward the driver’s side fender, or toward the engine. That doesn’t affect performance, but it does affect packaging. We also offer proportioning valves that you can mount right to the master cylinder itself for a clean look, or you can run a remote proportioning valve. In some high performance and racing applications, folks will run it into the car so they have a cockpit-adjustable proportioning valve.”

Click Here Now to see Baer Remaster master cylinders

Baer offers billet-aluminum and OE style cast-iron master cylinders, but it's largely an aesthetic choice. Many car owners choose Baer's billet-aluminum Remaster master cylinders, shown here, which are available in a wide range of colors and finishes.

A proportioning valve controls how quickly line pressure increases to the rear brakes, which can be useful when dialing in a setup where the brakes are often used near 100% of their stopping capability, such as in an autocross or road course-tuned performance vehicle.

Fowler says that while there are variables that can help determine the correct master cylinder bore size for an application, there are some subjective elements to it as well.

“When I’m sizing a master cylinder for someone, I also ask about who is going to be driving the car. Is it just the person you’re talking to, or is their wife or their grandma going to drive it too? One person may be comfortable with a firm pedal that has less travel and prefer to modulate clamping force with the pressure that they’re applying to the pedal, but grandma might not be able to create enough force on the pedal to do that. In that case, it might be better to go with a slightly smaller master cylinder to reduce the required effort a bit.”

See Baer's line of adjustable proportioning valves now

An adjustable proportioning valve allows you to tailor the exact balance of front and rear braking force. Although this component is necessary to get effective braking in any vehicle, it's especially important for vehicles that frequently use close to 100% of their braking capability, such as cars used for autocross or road racing.

A residual valve should also be considered on projects where the master cylinder is going to be mounted especially low in the vehicle.

“If you have an older street rod, or something where the master cylinder is mounted underneath the floor, that’s when you need to start looking at residual valves to prevent fluid bleed-back,” he says. “It’s generally not an issue if the master cylinder is mounted above the calipers and you’re running four-wheel disc brakes. But if that’s not the case, and the master cylinder is mounted down on the frame – which is something we also see with some older pickups – there’s a potential for the fluid to bleed back into the master cylinder from the calipers when your foot is off the brake pedal, and that creates a long pedal. The residual valve basically holds a bit of line pressure on the calipers at all times. It’s not enough that it’s going to create brake drag, but it will prevent that long pedal travel in those situations.”

The choice of whether or not to add a booster to your brake system mainly comes down to what you want the pedal feel to be like. But it can also be a matter of packaging and aesthetics. While this vintage truck has enough room to allow practically any master cylinder and booster combination, the manual system shown here looks considerably cleaner without the bulk of a booster behind the master cylinder.

Selecting the Right Brake Booster

The booster’s job in a brake system is to provide braking assistance by effectively creating additional pedal pressure. Depending on the type of booster used, this is accomplished either by internal diaphragms in the booster housing, or through hydraulic pressure.

“So, for example, it might require 100 pounds of leg pressure to stop a particular vehicle with a manual brake setup,” says Fowler. “But if a brake booster is added to that setup, it might only require 50 pounds of leg pressure to create the same amount of clamping force because the booster is providing the other 50 pounds of effort.”

Although vintage cars in general are thought to have loads of room under the hood, a pony car like this early Mustang gets crowded in a hurry when you factor in shock towers, aftermarket braces, and other impediments. Baer Remaster master cylinders are typically somewhat smaller than comparable OE units, making them a great choice where space is at a premium.

Fowler tells us that the first step toward choosing the right booster for a vehicle is an assessment of how much vacuum the engine creates. “Generally speaking, you need 18 inches of vacuum at idle to operate a vacuum brake booster. That’s usually not an issue with milder engines, but if you’ve got a radical camshaft in it, the engine may not be generating enough vacuum on its own to operate a vacuum booster.”

In terms of sizing, going bigger doesn’t have any downside from a performance standpoint, but packaging can become an issue.

“It comes down to what you can fit in there – the bigger the diaphragm diameter, or the higher number of internal diaphragms, the better. It just equates to more assistance. However, it’s important to be aware that a 9-inch dual-diaphragm booster will provide more assistance than an 11-inch single-diaphragm booster will. With vacuum boosters, it’s all about surface area.”

Optimum braking performance comes when the master cylinder and booster are part of a carefully integrated system. For that reason, the best time to choose a master cylinder and booster are when you're upgrading the rest of the system. Baer representatives are happy to guide you through the process and help you choose the right components for your intended application.

In situations where the engine doesn’t create at least 18 inches of vacuum at idle, you can use an auxiliary vacuum pump to get vacuum to the appropriate level. These pumps can generally be mounted in the engine bay or trunk – wherever there’s enough room. Operating off keyed ignition voltage, the pump will monitor vacuum levels to determine when to fire up and generate additional vacuum to keep brake system performance consistent.

“The other option is to use a Hydroboost system,” says Fowler. “This is a hydraulic booster that runs off the power steering pump to create assist. Hydroboost systems can be a very effective solution, but they’re a bit more involved and costly because of the lines and other things that are required to run them.” Hydroboost systems can also provide a tremendous amount of brake pressure assistance, and as a result, their use affects which size master cylinder would be appropriate. With that in mind, the choice between a vacuum booster or a Hydroboost system should ideally be made before selecting a master cylinder.

Autocross and track-day cars routinely put demands on brake systems that mild street cruisers don't see. The right master cylinder and booster for cars competing in these events is absolutely necessary to get not only the best braking capability, but the ideal pedal feel for each individual driver's unique preference as well.

Don’t Forget About Pedal Ratio

Fowler also points out that pedal ratio is often overlooked when selecting brake system components, but it has a significant effect on what parts are appropriate for a given application.

“You need two measurements to figure out the pedal ratio. Your brake pedal is mounted to the chassis and has a pivot point. To get the first dimension required, you measure from the pivot point to where your foot rests on the center of the brake pedal. The second dimension is from that pivot point to where the master cylinder – or the booster – pushrod attaches to the pedal. From there, you divide the second measurement by the first one, which gives you a pedal ratio.

“With manual brakes, we need a higher ratio because we don’t have that booster to give us assist,” continues Fowler. “So we need to increase mechanical advantage – usually 6 or 7 to 1 is appropriate for those applications. But with power-assisted brakes we don’t really need that mechanical advantage, so to avoid having a long pedal travel, we’ll decrease that pedal ratio down to somewhere around 3 or 4 to 1. That’ll reduce pedal travel, and the booster will provide the assistance required to get the appropriate clamping force.”

Although master cylinders and boosters perhaps don't have the visual appeal of other more external elements of a vehicle, these unsung components play a critical role in the overall effectiveness of your braking system.

As a result, the pedal ratio factors into what master cylinder bore size is correct for an application, along with booster sizing. “The pedal ratio can generally be determined by a vehicle’s platform. But with custom builds, this needs to be identified by the builder at the point where they’re choosing the type of brakes – assisted or non-assisted – and the pedal assembly that they’re going to use.”

While these tips should help get you pointed in the right direction, it’s important to note that brake system experts are just a phone call away.

“There’s a lot of different things that need to be taken into consideration when doing this, so we typically recommend that folks reach out to us directly to ensure they’re getting the right parts for their projects,” Fowler adds. “It definitely varies from car to car.”



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