Hot Rod Brake Upgrade
In many ways, hot rod builders are on their own when it comes to brake system component selection and system design. Unlike any of the production-based brake upgrades discussed this far, building a hot rod from scratch (or even from a kit) entails a unique collection of brake system considerations and constraints. With custom frames, powerful drivelines, modified bodies, and cut-and-paste suspension systems, there is little opportunity to learn from the original vehicle when the time comes to create your own hot rod brake system.
Yet the laws of physics still apply in these applications, making basic brake system design criteria just as important here as they were in Chapters 11, 12, and 13. The pages that follow will expose you to some of the most common design compromises, hardware considerations, and installation pitfalls that are either unique to, or exaggerated by, hot rod brake systems.
The subject of this hot rod brake upgrade project began life as a 1940 Ford pickup truck. Although it still possesses some token bits of the original body and frame, little else resembles what rolled off of the assembly line over 60 years ago. Custom roadster coachwork, independent front suspension, Chevy power under the hood, and a host of complex chassis changes only begin to describe the level of change this truck has experienced.
Although originally equipped with hydraulic brakes (mechanical brakes were only dropped the year before in 1939), the single-circuit master cylinder and four-wheel drum brake layout was dated at best. Therefore, as long as the rest of the truck was being significantly modified and updated, the brake system was upgraded as well.
Like most vehicles of this type, this hot rod was primarily designed for cruising along on a Sunday afternoon drive. Therefore, increased thermal capacity was not high on the list of needs or wants. At the same time, optimizing brake pedal feel was not critical for the truck’s intended use. In short, the most important brake system performance requirement was to slow the truck in a stable fashion during emergency-type events. Naturally, the parts had to look good (since that’s a big part of what hot rodding is all about), but performing a single stop quickly and reliably from a moderate speed was the most stringent brake system performance design target.
Front Brake Upgrade
The forward frame rails of the truck had already been modified to accept one of the most common independent front suspensions used in the hot rodding community. Lifted straight from a Ford Mustang II, this setup can be found hanging off the front end of countless hot rods today. In this application, the upper and lower control arms had already been replaced with custom tubular pieces, but the front uprights, bearings, and brake hardware were straight from the Ford parts bin.
Chosen more for availability and fitment than for thermal mass and effective radius, the front rotors measured a relatively small 9.0 inches in diameter and 0.9 inches in thickness. Although the straight vanes in the vented friction discs were not expected to be as efficient as curved vanes would have been, any front rotors would be more thermally robust that the stock front drums.
Like the front rotors, the front calipers were chosen more for convenience than for performance. These remanufactured single-piston calipers were based on the original Mustang II floating design. Their compact dimensions were the final consideration, as this would lead to flexibility in wheel selection later in the project.
Although a more modern Ford rear axle had been installed, in the interest of both time and money it was decided to leave the rear drum brakes in place. Of course they would receive a fresh coat of paint and fresh internal components, but with so little weight over the rear tires there was no reason to improve their performance. Even though it was still a pickup truck, its heavy hauling days were over.
Brake Pedal Considerations
In 1940, vacuum boosters were still several decades away from becoming mainstream brake system components. Therefore, because the truck was originally built without a vacuum booster (in other words, built with manual brakes), the brake pedal ratio was exceptionally high compared to conventional standards. Employing a pedal ratio of approximately 8.0:1, the stock brake pedal was required to swing through a relatively long arc as brake pressure was generated in the master cylinder.
Since the truck’s master cylinder was originally mounted beneath the floor of the vehicle, retrofitting a vacuum booster, even one with a relatively small diameter, would have been difficult at best. While upgrading to vacuum-assisted brakes would have been desirable from both a gain and pedal feel standpoint, in the end the stock manual pedal assembly was kept intact. In fact, with the exception of a thorough visual inspection and a fresh coat of paint, the pedal was lifted straight from 1940 without alteration.
Residual Pressure Valves
In many hot rod applications, the master cylinder is mounted below the hydraulic components at the wheel ends (disc brake calipers and drum brake wheel cylinders). Thanks to gravity, this arrangement results in brake fluid flowing from the wheel end components back to the master cylinder reservoir when it’s not pressurized. When the brakes are next applied, reduced pressure is available, accompanied by excessive brake pedal travel.
In order to prevent this phenomenon from occurring, a residual pressure valve can be installed in one or more of the brake lines. Acting much like a one-way flow restrictor at low pressures, these devices hold a constant amount of pressure at the wheel ends, preventing fluid from draining back to the master cylinder when the brakes are not applied.
Any time you’re using a low-mounted master cylinder, it’ll be necessary to install one residual pressure valve for each hydraulic circuit (blue and red). As long as they’re installed in the proper orientation, their physical location in the hydraulic system doesn’t matter. For ease of service and inspection, though, it makes sense to mount them where they’ll be readily accessible.
Based on the type of brake being used at a particular corner, different levels of residual pressure are appropriate to prevent drain back. Generally speaking, disc brakes require approximately 2 to 5 psi valves, while drum brakes, due mostly to their built-in retraction springs, work best with 10-to-15 psi valves.
If you’re unsure of the relationship between your master cylinder and the wheel end components, it’s best to be conservative and install a pair of valves. Even if drain back is not a problem, installing these valves does not pose any other negative performance impact to your vehicle’s brake system operation. When in doubt, it’s better to be safe than sorry.