A high-performance Mustang needs high-performance brakes. Better stopping capability allows you to travel at a higher speed for a longer distance and/or period of time because you can quickly slow the car enough to negotiate a turn without spinning out.
On the street, brakes are used very often even though they may not be used very hard. Certain changes to optimize a brake system for the street (longer wear with less dust, for example) conflict with those for the track but most characteristics that make a good track system also make a good street system. It generally comes down to what’s necessary for the intended purpose and the most cost-effective way to get it. Aesthetics and wheel clearance also figure in but they’re easy to accommodate.
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Early Mustangs used antiquated disc and drum brake systems. They are not up to performance use, or even regular street use, by today’s standards. This is due in part to the suspension design where, for example, deterioration and wear of the strut rod bushings can drastically affect brake performance and/ or cause the car to dart to one side under heavy braking.
Another big factor reducing braking capability is the prevalence of drum brakes. (I operate under the premise that nobody reading this book and wanting to improve the performance of their early Mustang considers keeping drum brakes on their car so I don’t discuss them. They simply aren’t up to the task nor are they the best when so many low-cost disc conversions exist.) Similarly, older OEM front disc brakes are not advisable, even the four-piston variety. The total capacity of these setups is limited by their piston sizes, rotor sizes, materials, and other properties. Furthermore, they’re not inexpensive or easy to find.
Aftermarket systems offer better performance at lower cost. In this chapter I discuss some options for upgrading your braking system to the level you need. I begin by discussing individual components in a typical brake system and what can be done to improve them for various situations.
Master Cylinders and Power Boosters
The first brake system component you run into beyond the firewall is the master cylinder or the power brake booster (if you have power brakes). For any vehicle that’s driven on the street you almost always want power brakes simply because of the fatigue factor. On a long trip, or even in heavy traffic on a long commute, power brakes reduce driver strain and can thus improve alertness. Power brakes generally also ensure you’re able to achieve line pressures high enough to obtain the shortest braking distances, even with excessive pedal pressure.
On the downside, power brakes add complexity, cost, weight, and take up room in the engine compartment. For racers and those who often drive their cars very aggressively power brakes can reduce pedal feel and modulation. This hinders the driver from braking consistently and with confidence. Most race cars lack power assist to provide the best possible pedal feel and modulation while also avoiding the extra complexity and weight. Power brakes are usually used in racing in longer, endurance-type races where the issue of driver fatigue is a concern. For all but the most extreme street vehicles a proper power brake system is the best choice for comfort and less fatigue on public roads with minimal loss of feel and/or control.
More than a few early Mustangs came equipped with manual (nonpower) brakes and a single-chamber master cylinder. From a performance and a safety standpoint this configuration leaves a lot to be desired and should never be considered for performance use.
A dual-chamber master cylinder is the only way to go and care must be taken to properly match the piston bore sizes inside the master cylinder to the brake hardware. A drum-brake master cylinder can almost never be used when converting to discs, for example. Depending on how many pistons there are in the calipers and what their dimensions are, there likely needs to be a change in the bore size. There’s no way to predict what may be needed in a given combination but, in general, the final parts must ensure the required line pressures are achieved in the front and rear brake circuits.
Line pressure should be checked with an appropriate gauge, not only when the system is first used but also periodically to help spot signs of a drop due to seal wear or other factors. Monitoring line pressure improves performance and safety because it can indicate the potential for a failure well before one actually occurs.
Under normal circumstances it’s not particularly difficult to find a suitable power booster if the car didn’t come with power brakes. Most OEM boosters provide sufficient assist for normal street use and most track use. Other than perhaps some heat shielding you need little else when installing a compatible booster.
When a very aggressive camshaft is used, there may not be sufficient engine vacuum available to provide sufficient power assist. There are several ways to resolve this with the easiest and most reliable being to use an electrically powered vacuum pump. Such devices are powered by the vehicle’s electrical system/alternator and automatically switch on when the brake booster vacuum is below a certain level. The pump runs until a specific level is reached and then shuts off. The pump cycles on and off automatically as dictated by the use of the braking system and other factors such as temperature and altitude. If engine compartment space is not available to fit a power booster and/ or extremely high line pressures are required, there’s another alternative: hydraulic assist.
Companies such as Power Brake Service have developed conversion kits based on the Bosch Hydro-Boost system used on many late-model vehicles. These systems are able to achieve the extremely high line pressures (1,200 to 2,000 psi at the calipers) needed when using fourwheel-disc brakes in heavier vehicles. They use the power steering pump to generate the pressure assist so no engine vacuum or external vacuum pump is needed. This reduces complexity and frees up significant underhood space.
The booster unit is remarkably compact, even smaller than the small-diameter vacuum boosters used in many special applications. The hydroboost unit is modest in circumference plus it’s short enough to mount between the master cylinder and the firewall.
The cost of these systems is reasonable even if their unique capabilities are not considered, usually on the same order as the small-diameter/racing vacuum booster kits. Hydroboost systems are immune from vacuum fluctuations, provide the most additional room underhood, and achieve the highest line pressures for master cylinders with bore diameters between 15/16 and 15 ⁄8 inches.
Brake Lines and Valves
Getting the brake fluid and pressure from the master cylinder to the calipers is done through a network of hard lines. These are usually made from galvanized steel. Companies such as Classic Tube offer kits for most early Mustangs made out of stainless steel, which look better and last longer; they can even be lighter. At the end of each hard line there is a flexible hose, which extends from the car body to the brake caliper. Because this hose must be able to move with the wheel it usually has some extra slack, which translates to a little extra fluid volume. Furthermore, because OEM hoses are made from reinforced rubber they tend to expand when pressure is applied.
These and other factors combine to delay the caliper’s response to pedal pressure, especially when high line pressures are experienced. This reduces the ability of the driver to get the best performance out of the brakes and could lead to premature hose failure. The solution is to use reinforced steel braided hoses instead of the OEM parts.
Such hoses exhibit virtually no expansion under pressure and thus provide a noticeable improvement in braking response while also providing much greater protection from potential damage and leaks caused by road debris, heat from headers/exhaust, etc. Most disc conversion kits include them. They’re also available from a wide variety of sources.
The proportioning valve is responsible for taking the fluid and pressure output from each chamber of the master cylinder and directing it to the appropriate calipers. It also acts as a safety valve in that it provides the driver with a warning if the pressure in one brake circuit falls out of balance with the other. This valve is required for virtually any all-disc system and is the same design as those found on production vehicles.
When changing from drums to discs or even changing the existing disc system it’s critical to use the correct proportioning valve for your hardware. Otherwise, you might end up with a very unbalanced system that locks up too easily or quickly or otherwise doesn’t perform properly. OEM components can often be used in most instances, though many aftermarket brake kits include new, superior valving products.
Between the proportioning valve and the caliper there may be a brake bias valve and/or a residual pressure check valve. The former is used to adjust the line pressure going to the rear brakes to help prevent their premature lockup. This can occur when the rear of the vehicle becomes unloaded during hard braking, especially if there are significant differences in the caliper, rotor, piston, tire, and wheel sizes. Such valves are manually adjusted as necessary and can even be used to compensate for tire wear and other factors during an event if they’re readily accessible to the driver or crew.
In some cases a residual pressure check valve may also be installed in the brake line to help prevent excessive pedal travel due to fluid draining back by gravity to a low-mounted master cylinder. By ensuring the line is “primed” the brakes respond more quickly to pedal inputs and are easier for the driver to modulate. The valves for disc-brake systems are rated very low, about 2 pounds, because calipers are not subject to return spring pressure.
For the most part, the calipers in an OEM front disc-brake package aren’t up to high-performance driving. The single-piston caliper versions are fine for a daily driver that occasionally sees some enthusiastic driving. Their sliding-piston design is inherently less stable plus the rotors usually aren’t big enough for extreme use. Upgrading the rear brakes to discs provides a similar improvement in braking performance, especially in the wet. Still, the nose-heavy design of the Mustang coupled with lots of forward weight transfer under braking can overwhelm the stock front brakes while the rears unload and are unable to work with maximum effectiveness.
A somewhat rare option found on early Mustangs is OEM fourpiston front brakes. This is a significant improvement over the normal front disc option because it’s a fixed (not sliding/ floating) design. Also there are four pistons (instead of one) to better distribute the higher clamping pressure over the pads.
A fixed-caliper design is superior to a floating/sliding single-piston caliper because the brake pads are pushed more squarely and evenly against sides of the rotor. This design can make for very quick pad changes because only two easily accessible bolts (or clips) need to be removed. These brakes are suitable for mild track use with the proper pads and brake fluid. They’re a relatively rare option and not really something you’d want to convert to or pay extra for.
Aftermarket front brake kits generally follow the same basic design as OEM four-piston calipers. Unlike OEM parts, however, aftermarket kits are offered with larger pistons in stronger calipers, which act on larger rotors with other features to improve performance. Kits intended for more-extreme use include calipers with six or even eight pistons in much larger, reinforced calipers, which may be a “monoblock” design machined from a single block of metal. Many even have pistons, which are staggered in size to help ensure more uniform pad wear.
Any of the components for high-performance driving are inevitably much, much stronger and lighter than their OEM counterparts while providing incredibly better braking performance due to their superior designs, materials, and manufacturing processes. Pad choice is critical to achieving this improved performance.
Rear Calipers and Parking Brakes
The situation for the rear is less complicated than for the front because the back of the car plays less of a role in braking, especially on a street-driven car. It’s rare to see calipers with more than one or two pistons each at the rear. Furthermore, the need for a parking brake on a street-driven vehicle can make more than two pistons less practical from a packaging and cost perspective.
In any event, converting from drums to discs almost always requires the removal of the axles, though some kits may include a split mounting flange to avoid this. The rear brake setup tends to be a simplified and scaled-down setup compared to the front with the primary difference being the parking brake.
Several popular designs of parking brakes are available. Each has its own pros and cons. All follow the OEM approach of adding a cable-controlled actuation system to supplement the hydraulic actuation. With a sliding/floating caliper it’s simple to have the cable action pull on the sliding portion of the caliper. There are internal and external designs to do this with the former being simpler while the latter tends to provide greater clamping force at the expense of added complexity and a less-straightforward installation/adjustment process. Either works well enough if the kit is properly designed and installed.
A new approach has recently become popular primarily due to its ability to provide even greater holding power while keeping the caliper design simple. This approach removes the parking brake function from the caliper entirely and instead incorporates a small drum-style brake into the rotor. This allows a standard caliper to be used instead of one that’s more complicated and likely less effective due to the integral parking brake. By separating the parking brake function from the caliper it can be optimized. The downside is there are extra components, cost, and a unique rotor design.
A final consideration is the interface/compatibility with the OEM parking brake cable. If you’re converting from drums you need new cables because the ends don’t match. Most kits include new cables if they’re needed. Companies such as Lokar offer a wide variety of modelspecific and universal products.
Rotors and Pads
Disc-brake rotors have improved dramatically over the years primarily as a result of much better materials and greatly improved manufacturing techniques. Make sure you’re getting high-quality rotors from a reputable US manufacturer (Baer, SSBC, Hawk, Stop Tech, Performance Friction, Raybestos, Bendix, etc.), not an inexpensive copy that can cost you more in the end. Premium foreign brands (such as Brembo, Alcon, and Disc Brakes Australia) have proven themselves in the market and on the track.
Besides basic compatibility with your vehicle you need to consider rotor diameter. A larger-diameter rotor provides more swept area/ braking surface and thus more ultimate braking capability (all things being equal).
However, you can go too big and increase weight and cost for no benefit. This may be for aesthetic purposes (such as with a larger wheel) but it’s impractical from a functional standpoint. You should only get as large a brake as is necessary, no bigger.
Thinner, solid brakes may be acceptable on the rear (only) of a daily driver but should never be considered for use on the front or with any higher-performance vehicle. They simply can’t take the heat of extreme use.
Thicker, vented rotors allow cooling air to pass through the rotor to reduce temperatures and provide more “thermal mass.” This allows them to absorb heat at a greater rate over a longer period of time before their temperature becomes excessive. Virtually all pro/racing rotors (except for drag racing and a few others) are vented because they work better.
Only vented rotors should be considered for any high-performance application. Solid rear discs may be okay when converting from drums.
Crossdrilling or Slotting
The next thing to consider is whether you want plain rotors or those with slots and/or holes. These should only be considered when the rotor is of known, good quality because these machining operations can cause problems with inferior rotors. Cracks tend to form around the machined areas unless the correct material and manufacturing processes (heat treatment, surface treatment, etc.) are used.
Crossdrilled holes have been used on production vehicles. They help evacuate the gases formed from the pads reaching higher temperatures. On a solid rotor these gases can build up between the pad and the rotor, thus reducing braking power.
Slotting performs the same function as crossdrilling yet it does so with less structural weakening. The gases can still escape, though not as easily. Slotting does, however, provide even more initial “bite” than crossdrilling. Slots have sharp edges (versus the radii of holes) that grab the pad more aggressively and make the brakes easier and quicker to modulate under hard-driving conditions.
The two styles have other pros and cons, not to mention their appeal (or lack thereof) from an aesthetic standpoint. One way to resolve this quandary is to use a mix of both. The slots give you initial bite while the holes evacuate the gases, save a little weight, and even help cooling if you run brake ducts to the rotors.
Less-expensive rotors tend to be single-piece units, like most OEM rotors. Racing and high-performance rotors tend to be two-piece or multipiece designs. Using a separate center section allows a lighter material such as aluminum to replace the iron of a single-piece rotor. This saves a significant amount of unsprung weight, which can aid wheel control and improve handling. A floating-style disc is even better at compensating for the expansion of the rotor as it heats up. This design allows the disc to grow with less restriction imposed by the center “hat,” thus helping to reduce distortion and warping. This keeps the whole rotor more dimensionally stable, which in turn, enhances the overall braking capability. This design is more expensive to produce but the advantages are clear if you need very extreme braking power. The rotors and/or hats must be properly treated/plated to prevent corrososion (for aesthetics) and to prevent rust from getting on the brake pads.
In general, pad choice is a compromise between long life and high grip. Softer pads deliver better performance and shorter life than hard pads. The balance you choose depends on how you use the vehicle.
Owners of street and track cars often have a set of street pads and a set of track pads. You want to be able to take full advantage of the sticky tires you may be installing when you run at the track. You don’t get the extra braking power the sticky tires can provide if you don’t have pads capable of locking up the tires. You need to have this extra braking power to get the best performance. It’s up to the driver to use this power properly but you can’t “threshold brake” if you can’t reach the threshold!
Vehicle Type 1: Daily Driver
For a daily driver the emphasis is on lower cost, reliability, and a mild performance increase over stock. Because the OEM system on these cars is much older technology this isn’t hard to accomplish.
The front rotors should be a single-piece type of about 11 to 12 inches in diameter with a single- or dual-piston caliper. Many such systems exist for very reasonable cost; some don’t even require that you change the front spindles. Such a system generally allows you to keep using the OEM 14- or 15-inch wheels, if you like, and provides a very significant increase in braking power with great reliability, long wear, and easily obtainable replacement pads.
The improvements in materials and processes plus the conversion to discs are significant benefits. When combined with larger rotors and better pads you see a big improvement in braking, especially with grippy tires.
At the rear you should go with an OEM single-piston sliding caliper that is low in profile with a 10- or 11-inch (preferably vented) rotor.
A Currie Enterprises kit, for example, takes the OEM rear disc brake system from an early 1990s Thunderbird and adapts it to an 8- or 9-inch axle. You get the benefit of a proven OEM system with easily found replacement pads for a relatively low cost. Currie includes new axle bearings, wheel studs, hub spacers, and parking brake cables to make the job easier. Combined with an upgraded front setup, this Currie rear kit helps dramatically improve braking performance while providing long life and smooth operation.
With a combination like that, a Hawk Premium Ceramic/OES pad or similar is the recommended choice. It provides better stopping power than OEM pads with very low dusting and low wear. These pads are designed to minimize brake noise and wear plus they carry a lifetime warranty. You can drive the car every day yet still take it to an autocross event or short-duration track day without having brake issues or swapping in other pads.
Vehicle Type 2: High-Performance Street Car
This car is driven more aggressively (but not necessarily as often) on the street while also seeing occasional track-day events. This requires changes to the size and type of brake components.
At the front the common choice is an aftermarket four-piston fixed caliper on a one- or two-piece rotor of about 11 or 12 inches in diameter. It’s desirable to go with slotted rotors for a better bite with some extra cooling/outgasing relief. Power brakes are strongly recommended, as are braided-steel brake lines. A manually adjustable brake bias valve is also highly desirable because it allows you to better optimize the brake balance at the track yet return it to normal for the drive home.
The rear brakes aren’t much different from those on a daily driver except that the rotors are likely a bit larger (11 or 12 inches) and the rear calipers are more likely fixed instead of sliding. The use of proper hub-centering rings is just as important here as it is with the other vehicle types: to ensure the rotor is correctly located with minimal runout.
The common pad choice is a Hawk HPS for the street and even some of the more moderate track uses. It provides considerably higher friction levels with only a minor increase in dusting, wear, and noise. Wear is still quite good for the pad and the rotor while the OEMstyle noise dampening features are retained. They cost a bit more but are well worth it and can realistically be used for the street and on the track if the car isn’t too extreme.
When conditions dictate, HP Plus pads (or similar) can be used all around at the track or just on the front if the rear brakes aren’t very heavily loaded. These pads provide even more stopping power while still being usable on the street. They aren’t very well suited for prolonged street use, however, because they tend to dust noticeably more, wear faster, and are noisier than the HPS versions. They also need more grippy tires.
Vehicle Type 3: Streetable Track-Day/Drag Strip Car
This vehicle type is a Jekyll and Hyde; it turns into a beast at the track/strip. Tires and brake pads are swapped, suspension settings are changed, some weight may be removed, maybe the engine gets a different tune; you get the idea. You can still drive to the event but not the way the car is normally set up: you optimize it for the event.
You want a braking system that performs as needed on the street yet has the reserve capacity to handle extreme (though not professional level) use at the track. For the front most owners go with at least a 12- or 13-inch two-piece slotted, crossdrilled, and vented rotor with a fixed caliper boasting four or more pistons. The optimum combination depends on many factors such as vehicle weight, degree of suspension modification, and type of duty the car will see.
You also want to plumb a manual brake-bias valve into the cockpit so you can adjust brake balance on the fly during the event. A hydroboost brake booster is a prime candidate for this scenario because of its high line pressures, compact size, and independence from engine vacuum. You need to use a racing-grade DOT4 brake fluid with a sufficiently high temperature rating. You surely want to consider running cooling ducts from the front of the car to the center of the front rotors to keep them from overheating on the track.
The rear setup should most likely feature a fixed four-piston caliper on at least an 11- or 12-inch two-piece vented, slotted, and crossdrilled rotor. A bigger rear rotor may be feasible in a heavier car if the suspension minimizes dive under braking and the tires can support it.
The parking brake is critical because the pads likely have less grip when cold even though they’re better when hot. Therefore, one of the inner drum-type parking brakes is best but other types can also be acceptable depending on how much you use the parking brake, the weight of the car, whether you park on hills, etc.
For this vehicle type the pad choice is more critical and more flexible. Due to the extremely high heat this setup can generate only an HP Plus pad or a race compound, such as the DTC-30 (or possibly something even more aggressive), should be the choice on the track. You can keep the HP Plus pads all the time if you don’t drive the car that often and you don’t mind the extra dust, noise, and (potential) wear. The HP Plus needs to operate in warmer weather and is not suited for colder climates. If you don’t use extreme braking on the street you can use the HPS for minimal dusting, longer wear, and less noise.
When you go to the drag strip other changes may be appropriate at this level. Because repeated stops are not part of the drag-strip duty cycle, the brake setup can be lighter and less dependent on cooling under extreme conditions. If the vehicle is still used on the street, however, the guiding principle becomes maximum performance on the street with the reserve capacity necessary for high-speed stops with lots of time for cooling at the strip.
A four-piston fixed caliper in the front and the back (or maybe even a smaller one in the rear if you don’t need much brake on the street) should work well. The rotors should be the vented two-piece type with slots and drilling, though their diameter likely does not need to be as large as for a high-performance street car.
Pad choice can be a bit more extreme for the strip where you may want to use a dedicated race pad if the HP Plus or DTC-30 doesn’t heat up fast enough. The race pads can be swapped in at the track or you can drive to the track with them already installed. Just remember to take them out when you get back home or you’ll see a lot of dust plus you may damage the rotors.
Some drag racers use minimal front brakes and a dual-caliper setup at the rear when they have huge rear tires, a parachute, and wheelie bars. It may also be a requirement in certain classes to have a dual-caliper brake setup on the rear axle, especially when a transmission brake is used to hold/launch the car. Such a setup isn’t advisable or necessary on the street and only benefits extremely modified cars on the strip.
Written by Frank Bohanan and Posted with Permission of CarTechBooks