We typically turn big tires on our 4WDs and when it comes time to slow down or stop, it's good to have a braking system that can handle the rolling inertia of that wheel and tire mass. That action starts with the mechanical movement of your brake pedal that actuates the brake master cylinder mounted to the firewall. From here, hydraulic fluid is pushed out through the connecting fluid lines to your calipers and/or brake cylinders at each wheel.
An automotive braking system is a hydraulic system where fluid pressure is transmitted to each corner of the vehicle to apply brake pedal force. This force is tremendous, and mechanical leverage and the advantages of fluid pressure applied to surface area are used to multiply that advantage.
Under your dash is a brake pedal that swings from a pivot point. Two lever arm locations hang from this pivot point. The first is a short arm (or sometimes a connecting point higher on the brake pedal arm) that connects to a small steel rod that enters the master cylinder through the firewall. The second is the brake pedal where you place your foot. The brake pedal is much longer than the master cylinder lever arm. This is the first mechanical advantage in the brake system. The ratio of this mechanical multiplier may be about four times as we trade movement at the pedal for about a quarter of the movement and four times the force at the master cylinder rod. The goal is to build tremendous pressure at the brake cylinders to apply the brake pads without requiring too much legwork.
The rod then passes through the firewall and may enter the master cylinder unboosted, or may enter a vacuum or hydraulic power booster cylinder. The majority of vehicles use a vacuum-assist booster that is powered off the intake vacuum of the engine. Modern brake master cylinders contain two pistons to feed two separate fluid lines. This allows the four brakes to be separated into two sets of two for safety reasons. Should one fluid line ever fail, you are still left with two brakes intact.
A vacuum booster mounted to the firewall works by using the energy of the engine intake vacuum (or a vacuum pump) to help reduce the braking effort your foot must apply. The booster consists of a large sheetmetal canister that is mounted to the firewall between the brake pedal rod and the brake master cylinder. The force of the input rod is exerted on a bladder that essentially has ambient pressure on the input side, but is under a vacuum on the backside. This differential pressure is used to assist the action of the brake rods. A valve within the booster prevents the pressure differential from developing until at least some pressure is placed on the brake pedal by the driver. This prevents any slight application or dragging of the brakes when they are not in use.
A hydraulic booster works in much the same way as a vacuum booster to help provide mechanical advantage to the master cylinder. It too bolts between the firewall and the master cylinder. In this case, the source of power is the power steering pump. It provides fluid pressure to the hydraulic booster that is used to help push the master cylinder rod.
With both vacuum and hydraulic boosters, the input rod from the brake pedal enters the booster and is intercepted and routed through the boost mechanism. However, in both cases, should the boost system fail, the pedal input rod will still contact the input rod at the master cylinder to operate the master correctly. This fully manual fail-safe mode allows the brakes to still operate unboosted, should the boost system fail.
There are a number of aftermarket suppliers that offer disc-brake conversions for rear axles. Disc brakes offer several advantages over drum brakes. Since the friction surface (rotor) is exposed, disc brakes cool much quicker than drum brakes. There is also no problem with the friction surface deflecting as there is a pad pushing on each side of the rotor. Disc brakes do not retain water as do drum brakes. Disc brakes are also lighter in weight, easier to service, and brake more consistently, so are less likely to pull to one side.
Drum brakes have several disadvantages. First, when used extensively, they may be prone to brake fade as the iron drum expands and wants to move away from the brake shoes. Drum brakes also tend to retain water and mud and are not as self-cleaning as disc brakes. There is one disadvantage to disc brakes when compared to drum brakes. Self-energizing drum brakes are actually designed to pull the drum brake shoe into the drum when the brakes are applied. This offers the driver some mechanical advantage. Disc brake pads are flat and run entirely parallel to the rotor surface. There is no such self-energizing action with discs. To compensate, larger calipers and a master cylinder booster may be added to the system.
There are three varieties of disc brake calipers: fixed calipers, floating calipers, and sliding calipers. A fixed caliper has a piston, or pistons, on each side of the rotor. The caliper is solidly bolted to the steering knuckle or other caliper mount. Pistons move equally from each side of the rotor to apply pressure directly on both sides of the rotor. Most fixed calipers are assembled from two castings that are bolted together. Passages within the two halves allow brake fluid to flow to pistons on both sides. This style of caliper often offers the best braking performance, but is also a more expensive option due to the greater complexity and higher parts count.
In contrast, a floating caliper has a single large piston (or two smaller ones in some cases) located on the inboard side of the caliper. Since force must be applied equally to both sides of the rotor, the floating caliper must be able to actuate both pads. The caliper is bolted to the mount with special pins, and the caliper slides back and forth on sleeves or bushings. The inboard pad sits directly on the piston. The outboard pad sits within the caliper frame. When the brakes are applied, the piston moves outward pushing the inboard pad into the rotor. Simultaneously, the force exerted backwards from the piston onto the caliper frame causes the frame to move inboard, bringing the outboard pad into contact with the rotor. In this way, both pads press on the rotor. One advantage of a floating caliper is that the moving caliper design can better tolerate the movement of a warped rotor than a fixed caliper can.
A sliding caliper operates in much the same way as a floating caliper. Instead of using a set of pins, a sliding caliper rides in a set of machined guides. The guides allow for the lateral movement of the caliper.
Disc calipers are inherently self-adjusting by design. Each piston is built to fit with close tolerance to its bore. An O-ring seal and dust boot are also fitted to each piston. The action of the O-ring is such that it allows the piston to slide through it as the pads wear so that the brake pedal remains firm throughout the life of the pads. However, the O-ring also provides a slight retraction movement to keep the pads pulled just off the surface of the rotor. In this way, the seal is constantly keeping the caliper adjusted as the pads wear.
Disc Brake Upgrades
Disc/drum systems offer better braking performance than drum/drum systems, and disc/disc performance surpasses that of disc/drum systems. When starting with an older vehicle that came from the factory with drum front brakes you can often swap to more modern factory disc components. You can do the research and hit the junkyard and parts store to assemble all the OEM parts, or you can order a complete kit from an aftermarket supplier.
More often than not, rear brakes on a 4WD come from the factory in drum form. While many modern self-energizing drum brakes are quite efficient, rear disc brakes remain desirable for several reasons. Rear disc brakes require no adjustment, are more consistent, and are much more immune to mud and sand retention problems as compared to drum brakes. For many vehicles, it is possible to convert from rear drum brakes to rear discs. Kits are available for a wide variety of applications and very often allow use of calipers that retain the stock emergency brake function.
It's also common to perform disc brake conversions on custom or full-floater rear axles. Caliper brackets such as these Dana 60 versions from Ballistic Fabrication can be purchased and welded (or sometimes bolted) to your axle housing. Conversions such as this are most often done using floating pin calipers as they require the least precision in mounting and are a common caliper type.
When swapping axles and other driveline components you may end up swapping brake calipers/wheel cylinders to facilitate an upgrade to beefier parts. As calipers/wheel cylinders grow in diameter, they require greater fluid flow for the same pad or shoe travel at the wheel. If larger brake parts are installed at the axles, the stock master cylinder will require greater pedal movement for the same brake pad or shoe actuation.
The master cylinder is sized from the factory to offer comfortable pedal travel with ample margin keeping the pedal from bottoming on the floorboard. Installing larger bore calipers with the same master cylinder will result in longer pedal travel to apply the brakes, or may require a second pump of the pedal to stop well, leaving you little or no pedal travel margin.
If the pedal travel becomes too great, it may be necessary to swap to a larger bore master cylinder to feed more fluid to the calipers. There is no added advantage to increasing the master cylinder bore, and with all other components equal, a larger bore master will actually result in less mechanical advantage in the system. It's best to go larger only if you lack enough fluid movement (i.e. too long of pedal travel).
The fluid flow and mechanical advantage offered are directly dependent on the bore size (surface area) of the master cylinder piston. Let's consider the changes that occur if a 7/8-inch master cylinder is replaced with a 1-inch bore master cylinder. The piston surface area increases by about 30 percent in this case. The larger master cylinder will move about 30 percent more fluid with the same pedal travel, but will require a similar increase in force at the pedal to obtain the same braking pressure.
A similar calculation is valid on the caliper side with a larger piston providing greater mechanical advantage. For a four-piston fixed caliper, we would use the surface area of two pistons, not four. The pistons on both sides of a fixed caliper need to move only about one half the distance the one piston on a floating caliper does.
Using a master cylinder size that provides reasonable pedal travel and maximum mechanical advantage will optimize your brake swap. Overall, we're trading a large pedal movement with light applied force to very small brake pad movement with large applied force. We're also using a relatively small bore master cylinder with long piston stroke to feed larger bore calipers to make a much shorter piston movement.
Depending on your swap specifics you may or may not have sufficient room to fit a different brake booster, so check your diameter and available space carefully. There can be a wide range of master cylinders that may work for your swap depending on the flange mount and bore diameter needed. You may also find the fatter cast iron variety master or the thinner aluminum one with the plastic reservoir sitting atop. One shape may fit better than the other in your engine bay.
Whenever you modify your braking system, such as the addition of rear disc brakes, it may be necessary to add a manual brake proportioning valve to the rear brake line. This can allow you to manually adjust the pressure to the rear brakes in a case where the rear discs may be over-powering and unbalanced when compared to your front discs. A manual proportioning valve such as this unit from Wilwood can be used to decrease the braking force of the rear discs.
Some brake line adaptation may be necessary. If so, there are all manner of adapter fittings available, even metric to SAE, and vice versa. You can also buy steel lines with metric or SAE ends, and a flared end can be swapped as needed if you have double flaring tools.
Another option when it comes to adapting brake lines is that specialty companies, such as Crown Performance Products, can custom build stainless braided flex lines to any length and with any terminating end that may be needed. These can be used to plumb the calipers on an axle, or to plumb from the master cylinder to an axle.
Where brake lines come off the master cylinder and route down to points on the frame rail, you'll typically find relief coils. These are there to provide for some movement between the body and frame without having the brake line stress, and fatigue or crack. Also, When routing your own brake lines be mindful of securing them to the body or frame every few feet or so. When in use, they are subject to constant flex and vibration. The metal lines can fatigue and fracture over time if not properly staked. Needless to say, it's prudent to take care when working on or modifying your brake system. If you're not confident of doing a reliable job, refer to a competent mechanic for a safe installation.