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Best Brake Boosters for Better Braking

6/2/2021
Updating Your Muscle Car, Hot Rod, or Truck to Power Brakes Will Make Every Drive More Enjoyable

The vacuum-assist brake booster we commonly see utilized to reduce brake pedal effort and increase braking performance was invented in the late 1920s and initially manufactured through the Robert Bosch company. Obviously, brake boosters were not implemented in domestic vehicles until they went away from mechanical brakes, which was the mid-to late 1930s for GM and Ford. Manufacturers added the power brake booster to their braking systems to provide a softer, more manageable, pedal feel for their customers with the bonus of the device making the braking system more effective and reducing stopping distances. It was a rare option unless it was a higher end vehicle of the day, but power drum brakes became more common in the 1960s, eventually leading to power disc/drum or power four-wheel disc being standard on most cars in the late 1970s and early 1980s. While today’s cars often have electric brakes or hydraulically-boosted brake systems, the venerable vacuum brake booster is still widely used today on new cars and trucks. The brake booster is simple, relatively easy to convert a vehicle to, and not at all expensive, making it a great safety upgrade for hot rods, muscle cars, kit cars, and more.

How Does a Brake Booster Work?

The brake booster housing contains a large rubber diaphragm inside with a series of springs and pistons/sleeves. Engine vacuum (or an auxiliary electric vacuum pump) provides a vacuum signal to one side of the diaphragm and is prevented from escaping via a brake booster check valve. The other side of the vacuum brake booster is opened to atmospheric pressure when the brake pedal is depressed, allowing the vacuum signal on the other side of the diaphragm to aid the diaphragm and booster pushrod. This provides an extra level of assist to the master cylinder bolted to the brake booster, providing better braking with less pedal effort.

Illustration courtesy of the Automotive History Preservation Society Digital Documents Library www.ahpsoc.org

A brake booster’s effectiveness is determined by the amount of vacuum supplied, the size of the diaphragm (diameter of the booster housing) and whether the booster uses a single or dual diaphragm design internally. We offer direct replacement brake boosters for many muscle cars and trucks, along with universal boosters and brake booster/master cylinder kits for hot rods, both firewall and frame mounted, to work with just about any project vehicle you can imagine.

What Is the Difference Between A Single and Dual Diaphragm Booster?

The obvious answer is that one brake booster has a single rubber diaphragm inside the metal shell and the other has two. But it goes much further than that. A single diaphragm booster has a shallower overall depth versus the dual diaphragm unit (when comparing same diameter shells), often providing much needed installation room in more compact frames or engine bays.

Single diaphragm brake boosters are commonly used for four-wheel drum or front disc/rear drum brake system configurations. Since drum brakes require a lower fluid pressure to actuate, the single diaphragm provides enough assistance. When upgrading to four-wheel disc brakes a dual diaphragm brake booster is recommended due to the higher fluid pressures required to move the multiple disc brake caliper pistons found at all four corners of the vehicle.

Shown here in this side-by-side comparison you can see the increased depth of the dual diaphragm brake booster (right) versus the single diaphragm brake booster on the left. This additional length, coupled with the installation of a typical tandem brake master cylinder, can create installation issues in both frame mount and firewall mount applications, so be sure to measure carefully.

Does Brake Booster Size Affect Stopping Ability?

There are several things that can affect a brake system’s ability to stop a vehicle in a safe and controlled manner. The brake booster diameter is certainly one of them. The larger the diameter of the brake booster shell the more surface area of the diaphragm used within the unit. This allows for a lower vacuum threshold to actuate the assist. Larger boosters can get away with needing only 14-17 in/hg of vacuum from your engine, whereas smaller boosters, like the popular 7-inch brake booster utilized in many conversion kits, can take up to 22 in/hg of vacuum to provide the same assist levels.

Considering your vehicle’s installation room, type of braking system (drum/drum, disc/drum, or disc/disc), and available engine vacuum to “power” the brake booster all must be weighed to come to a viable solution for your specific installation and braking needs. It is easy to say “go bigger” but that may not always be possible, especially in small hot rods that use frame mounted brake boosters/master cylinders.

If you are uncertain of your engine's vacuum output a basic vacuum gauge is the perfect tool to confirm your engine's vacuum numbers. It is a great tool for tuning carburetors, testing mechanical fuel pumps and more and should be an integral part of your toolbox "bag of tricks" if you do not have one already.

What if My Engine Does Not Create Enough Vacuum?

If you are limited on booster diameter or your engine does not produce enough vacuum signal, a brake booster vacuum pump is a great solution to ensure your power brake booster is receiving enough vacuum for it to function as intended. Remote electric vacuum pumps can be mounted just about anywhere, from the trunk to the engine bay, or inside a fender, or even directly to the frame crossmember under the car. These electric pumps simply need key-switched power and ground electrical connections and a suitable length of vacuum hose (or a combination of hard line and rubber hose end fittings) to connect it your power brake booster. Many electric vacuum pumps ship complete with vacuum switch, wiring harness, and more for an easy installation.

What is the Best Brake Booster for My Hot Rod or Muscle Car?

The easy answer here is the one that fits, but as enthusiasts we all know it is never that easy. Obviously beginning with the largest booster that will fit your application is a great start, but you also need to consider your brake system’s needs regarding single or dual diaphragm boosters, and the available vacuum your engine produces. If your engine is relatively stock and it produces 20 in/hg of vacuum you should be just fine with a 7-inch diameter booster, even if you do have room to go larger. However, if you have a performance camshaft and other engine modifications that reduce your available vacuum to only 16 in/hg, then that larger OE-style booster is most likely going to be mandatory unless you want to add a remote electric vacuum pump.

If converting to power brakes from an original manual setup you also must consider your stock brake pedal ratio and the master cylinder you are using to ensure you have safe and effective braking. For this very reason we highly recommend and offer complete power brake conversion kits that include a brake booster and master cylinder combo with the proper pedal ratio and mounting hardware/brackets included for many popular muscle cars, as well as universal brake booster and master cylinder combo kits for hot rods and trucks.

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