DRUM BRAKE EVOLUTION
To understand the how we must first discuss the why. Brake technology has made quite a few advances in the last 100 years, but even back in the 1940s, Ford adapted changes to the way brakes were designed, much to the resistance of Henry Ford.
Henry Ford was fond of his mechanical brakes, and his marketing touted "Solid steel from pedal to wheel". These mechanical rod operated brakes persisted until 1939, well behind Ford's competitors. The problem with the brakes is that they required frequent adjustment, and were just plain dangerous at speed. They featured 12 inch diameter drums that could easily put a car into a spin if one brake rod was tighter than another. High speed stops proved to be hair raising experiences, and many felt that the new hydraulic or "juice" brakes introduced in 1939 to the Ford product line was a step up in safety and performance.
The first and by far the most important design improvement was the change from rods to hydraulic fluid to actuate the brake shoes. Although the design was originally invented in 1924, Ford resisted utilizing the design until 1939, with the backing plates, shoes, and drum assembly that in many ways the is same design used on cars today. There were a few key design issues that took some years of production to be recognized and improved upon. Along with other idiosyncrasies, Ford's new adopted hydraulic design was a work in progress. The brakes required more pedal effort to stop the car than our modern drum brakes, a break in period, and even in some cases machine grinding to fit correctly.
Back in the 1940's and 1950's our hot rodding fore-fathers had limited resources and little access to sophisticated machinery, so often a "bolt on solution" was the most popular. From the way Ford designed their cars, these brake alternatives were just that.
The second major design improvement was the way the shoes were mounted on the backing plate. The 1940 brake shoes pivoted on a pin at the bottom and where actuated by a slave cylinder at the top. This design is known as "single leading shoe" or "leading and trailing shoe brakes". When brake force is applied, both shoes pivot at the bottom attachment bolt and press against the drum with equal pressure. Shoe contact tended to stay in one spot and the wear was uneven. Pedal effort was high because none of the rotational force was used to help make the shoes press harder to the drum surface.
Living with the 1939 Ford brakes
To improve shoe to drum contact, the 1940 design utilized a pair of cams on the shoe pivots that moved each shoe in or out to eliminate high spots. Either lock nuts on the backing plate side of the brakes, or spring tension bolts allowed for adjustments to be made.
These spring tensioned bolt adjusters tend to resist staying in place, and the situation is even more aggravated if you chemically clean the backing plates for painting. Rust tended to create a resistance to turning of these spring bolts. The lower cam bolts on the design on the right tended to loosen as well, so when using the early Ford brakes periodic inspection is a must for optimal performance. Overall the design works well but requires maintenance.
A word about brake shoe grinding:
Brake grinding was common back in the day, as everyone was well aware that a little asbestos dust never hurt anybody! Perhaps no one thought of the disadvantage of grinding off of valuable braking material was a problem either. If the brakes where not ground down, they would eventually wear to the shape of the drums, but prior to that happening, the 1939 brakes had a nasty habit of grabbing and high spotting. This caused wheel lock at inopportune times, along with unpredictable braking. These features and disadvantages make the selection of the F100 truck brakes an easy choice.
Ford F100 pickup truck brake design
The F100 brakes are what is called a "Duo-Servo" design. This design works outperforms the 1939 leading edge design because it uses the rotation of the drums to cause the brake shoes to "twist" and press against the drum lining much harder. So much better in fact that they require less pedal effort and are less sensitive to high spotting. This also has the benefit of a more even brake shoe wear and longer shoe life. Let's look closer at why the "Duo-Servo" design works so much better than the 1939 design.
At first glance the F100 design looks much like the 1940 design, but on closer examination we see that the shoes are not mounted on a stationary pivot point, but rather "floating" on a pin and spring assembly. The floating feature allows the brake shoes to twist and wedge themselves against the drum increasing braking friction. Through this servo action the shoes transmit motion to each other, rather than just one lead shoe doing all the work.
Throughout the 1950's there was a revolution of sorts on brake development. Some of the larger more prestigious cars came equipped with this updated design. The major distinction was that the shoes now "floated" on the backing plates and took advantage of the rotation of the drum to grip harder. Ford recognized the advantage this would have on the heavier truck they produced during the early 1950's. The point not lost on performance enthusiasts. Even though the early brake drums were 12 inches in diameter, the 11 inch F100 and F1 design was far superior in performance.