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Brake Booster Buyers Guide

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Power brakes did not hit the mainstream automotive market until the mid-1960s. Before then, it was all legs, but power brakes made it a lot easier to bring your vehicle to a stop thanks to what is known as a power brake booster, or simply called a brake booster. Basically, power brake boosters do the hard work for you. It multiplies brake effort without any added pressure from your leg. It is simple how power brake boosters work, but it can be difficult to find just the right one you need for your ride. Thankfully, we have many options for power brake boosters from which to choose, including direct replacement, manual to power brake booster conversions, and universal brake boosters for hot rods and kit car fitments.

What Are Brake Boosters?

A power brake booster is the part of the braking system that applies power assist via engine vacuum. Vacuum is sourced from the engine or an electric vacuum pump and supplied to the brake booster housing. Inside the housing is a large diaphragm and a series of springs and pistons. The other side of the diaphragm sees atmospheric pressure when the brake pedal is applied, thereby providing an increased level of assist to the master cylinder that is bolted to the brake booster, which in turn applies pressure to the vehicle’s calipers and brake pads or shoes via the brake lines and the brake fluid within. The power brake booster is connected to the brake pedal, and in most applications, is visible in the engine compartment at the firewall. Some installations will find the brake booster and master cylinder mounted to the frame and below the floor of the vehicle and some custom applications have even located the brake booster within the vehicle’s dash area.

How Long Does A Brake Booster Last?

Brake boosters can last the life of the vehicle to which it is fitted. Since a brake booster is a contained unit, they are impervious to most outside conditions. However, brake booster life is affected in extremely dry climates, and/or if the vehicle it is in sits for a long time without use. A brake booster’s biggest enemy is dry rot, but with regular use, a brake booster can go 150,000 miles, and much more in the right climate.

Are All Brake Boosters the Same?

All brake boosters perform the same function. The difference between them is their size. For instance, if you need a universal brake booster for your custom build, they are available in different diameters, like the popular 7-inch brake boosters, or an 8-inch brake booster, and you can even dress up your engine bay by adding a chrome brake booster. So, if you have a packaging nightmare of an engine compartment, choosing the right size brake booster is of the utmost importance.

Then there are single and dual diaphragm brake boosters. A single diaphragm brake booster is ideal for drum brake systems, or a disc front/drum rear system. While dual diaphragm brake boosters work better with 4-wheel disc brake systems because that design produces a higher level of brake assist, no matter the packaging size. A dual diaphragm brake booster is generally preferred if there is sufficient installation room.

Recently the popularity of hydroboost brake boosters has risen. These hydroboost units are compact, easy to install, and engine vacuum is of no concern, which is great for radical camshafts, or individual runner manifolds that cannot provide a reliable vacuum signal. These hydroboost systems utilize the power assist from the hydraulic power steering system. The hydroboost is plumbed between the power steering pump and the steering gear or rack-and-pinion. The only drawback to these hydroboost systems is that they cost significantly more than a conventional power brake booster and require the installation of a power steering pump if the vehicle does not already have one, but the advantage these systems bring is, in many cases, worth the investment.

Does Brake Booster Size Matter?

Yes, the size of the brake booster matters, but only when it comes to available engine vacuum. Smaller universal brake boosters may work better for tighter confines, but they require more vacuum to operate properly. Without sufficient vacuum to the brake booster, braking performance suffers. You can recoup some of that vacuum by using a vacuum pump, but then that brings up packaging concerns for those with tight engine confines like street rods, as an example. The best solution is to use the largest booster diameter that will fit your application. 

How Do You Know When You Have A Defective Brake Booster?

The easiest way to tell if your brake booster is going out is that your brake pedal is either spongy feeling or rock hard with reduced braking results. If your brake pedal feel changes, and your vehicle’s pads and shoes are good, fluid level is where it is supposed to be, and you have no fluid leaks, you could have a failed brake booster. But before you swap out your brake booster, check the vacuum hose, and make sure it is still in good shape. A lack of vacuum to the booster will mimic the sign of a failed brake booster. If everything else within the braking system checks out, but your pedal feel has changed, and you have a loss in braking performance, it could be time for a new brake booster.

How Do You Test A Brake Booster?

First, check to make sure you have manifold vacuum, then test the brake booster’s check valve to make sure it is functioning properly. If you have the appropriate level of vacuum (roughly 17 In/Hg or more) and the booster’s check valve is working properly the next thing to test is the brake booster itself. With the engine off, apply pressure to the brake pedal, and then start your car. If your brake pedal drops, your brake booster is working properly. A second test can be performed as well. With the engine off, pump the brake pedal. Each time you pump the brakes, the brake pedal should become higher with each consecutive pump of the pedal. If the brake pedal stays at the same height with each press of the pedal it is likely you have a failed brake booster. A final test is with the engine running, press the brake pedal, and hold the pedal down. If the pedal rises any while holding foot pressure on the pedal your brake booster has failed. If the pedal stays at the same height, your booster is working properly.