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Mastering Mustang Brake Boosters with “Booster” Steven Potter

Back in 1988, during my days at Big-O Tires, a catchy phrase caught my eye on the reader board: "Can't stop, that's the brakes." Little did I know that phrase would stick with me throughout the rest of my career.

After years of working at Big-O Tires, gaining experience in front-end work, alignment, and most importantly, brakes, I moved to Power Brake Booster Exchange Inc. Here, I spent over 30 years – 20 under the mentorship of "Booster" Dewey – restoring brake boosters for sports, classic, and muscle cars. I eventually went on to purchase and take over the shop myself. 

During my time working with “Booster” Dewey, our focus was on classic cars rather than everyday vehicles. As a certified car enthusiast, this was right up my alley.

Over the years, I restored several vintage cars, which helped me better understand brake boosters. I familiarized myself with rod codes for different years and part numbers specific to certain cars. For instance, a GTO booster needs a different internal reaction spring than a Chevelle SS, and a GSX uses yet another type. 

Even within the same model year, like the 1969 Trans-Am, there are unique components like a special spring and reaction valve to simulate a "high-effort" manual brake feel. 

And while boosters can be interchangeable, alterations over time may not align with the original design by engineers, resulting in suboptimal performance.

The importance of choosing the right vacuum hose for your brake booster

Let's take some time to explore vacuum hoses in more detail. First, for most standard brake boosters, the correct vacuum hose size is 11/32. I've seen countless cars at shows and received calls from guys with problems caused by using a 3/8 fuel hose.

Using a vacuum hose specifically designed for this purpose is important, as using a fuel hose instead can lead to serious issues. Unlike vacuum hoses, fuel hoses are intended for pressure, not vacuum. When subjected to 20+ inches of vacuum, the fuel hose can collapse, cutting off the vacuum supply to the booster and resulting in hard brakes.

At the same time, a fuel hose is more prone to kinking than a vacuum hose with a thicker wall. While the inside diameter (ID) of the 11/32 vacuum hose is smaller than that of the 3/8 fuel hose, its larger outside diameter (OD) provides added rigidity, preventing kinking and collapse.

Vacuum hoses are clearly labeled as "vacuum hose”, while fuel lines are labeled as “fuel lines”. Using a fuel line instead of a vacuum hose will only lead to trouble, so it's essential to stick to the right hose size and type for peak performance.

A look at the evolution of Mustang brake boosters

Ford Mustangs have always been iconic, and I've enjoyed owning several over the years. From the outset, Ford offered various options, including power brakes, which were available right from the first models.

1964: 65 model year Mustang

Taking it all the way back to 1964, the brake booster at the time was a 6" Bendix single diaphragm unit mounted on a bracket. 

While it was a nice option, it used a single reservoir master cylinder and wasn't offered with disc brakes – not even on the Shelby GT's. 

The rod code (internal Bendix code stamped into the rod) for these units was initially "FA". It later changed to "MU" during production. Either code is correct for the 1964-1966 models. 

The check valve was positioned at the bottom of the front of the unit and pointed straight out. While these units weren't very powerful, they functioned well within the constraints of the engine bay of the first-generation Mustang.

1967: The Mustang upgrade

In 1967, the Mustang underwent big changes, becoming a lot more powerful. With extra room under the hood and new government requirements for dual reservoir master cylinders, Ford switched booster suppliers to Midland Ross. 

The new 7 ½" tandem diaphragm unit bolted directly to the firewall and packed considerably more power than its predecessor.

Unlike previous models, Midland boosters didn't use rod codes. Instead, they were identifiable by an aluminum tag with the unit's part number and series number, positioned at 10 o'clock on the band clamp. Some were marked with "Midland" or a 4200 number, indicating factory replacements.

The check valve of the Midland booster is at 12 o'clock. If it's at 10 o'clock, it's from Fairlane or Torino. These models sometimes get mixed up during rebuilding. 

Midland boosters needed a front "spacer plate" between the booster and master cylinder to work correctly.

For the 1967-1973 models, the brake pedal differed between manual and power setups. The power brake pedal was lower to the floor for better control.

It's important to note that using a non-power brake pedal with power brakes can damage the rear hub on the booster, so compatibility is an absolute must!

1969: Bendix boosters return

In 1969, Mustangs switched back to the trusty Bendix for their brake boosters. The Bendix tandem diaphragm boosters were used for the '69 and '70 models. 

They were easy to identify with their rod codes: MU1 for '69 and MU3 for '70. Unlike the previous Midland units, Bendix boosters were crimped together, and the check valve was at 10 o'clock. They didn't need a front spacer plate but did have a rear one secured with a nut.

These Bendix boosters were tough and used across most Mustangs, except for the Boss 429. This model needed a special truck booster because of its large heads. 

Like the Bendix booster, this truck booster had one big diaphragm instead of two. While reliable, it couldn't handle the heavy demands of driving hard, often leading to Bakelite hub failures. When rebuilding these boosters, many showed signs of wear, particularly the Bakelite hubs. But the replacement hubs were easy to find from other truck units. 

The 1970 model kept the same design as '69, except for a change in the activation rod. This rod went from a teardrop to a straight design, and a different pedal was used. The only exception to this was the 1970 Shelby, which stocked with the '69 setup.

1971: Bigger Mustangs, bigger boosters

In 1971, as Mustangs continued to grow, there was yet another change in the booster design. Sticking with Bendix, they replaced the tandem diaphragm with a single 11" unit. This new booster provided sufficient power for the larger vehicle.

Discover the Power Brake Booster Exchange advantage 

Step into the world of Power Brake Booster Exchange, where we excel in reviving vacuum brake boosters tailored for vintage and high-performance vehicles.

Benefit from three decades of expertise as our seasoned professionals handle your Midland and Bendix-style booster needs, whether it's for your beloved Mustang or Cougar.


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