What’s the first thing we as four-wheelers do with a new-to-us 4x4? We add bigger tires and aftermarket wheels, of course. We do this to change the look and improve off-road performance. Sometimes this modification also involves a leveling kit for a bit more space or perhaps the addition of a high-end custom suspension system to make more room for the tires and increase off-road performance. We may also add body armor, bumpers, winches, tire carriers, and other accessories that increase safety and utility. Before we know it, it’s time to regear the axles to compensate for the larger tires and add engine performance enhancers to help compensate for all those heavier components. It’s a vicious cycle, but we do all this in hopes that our 4x4 will take us to adventure, to the unknown, exploring the world around us and getting away from the fast pace of modern life.
One aspect of vehicle performance that all too often gets left in the dust is our 4x4s braking system. Larger tires and heavier wheels change the effective gear ratio of our trucks and increase the work that our brakes see during everyday use. Making our 4x4 heavier with all these additions taxes our trucks vital ability to stop. Gearing axles down and adding torque and horsepower all further push the factory braking system of your 4x4. What can we do to increase braking performance? Well, luckily there are a few things we can hone, twist, and prime for additional braking performance. The first step is to understand the important parts of the system and then we’ll touch on what you can do to help your truck perform at its best when you need to grab those binders.
Major Braking Components
Early on, a 4x4’s brake system master cylinder was little more than a reservoir and a pushrod with a plunger and some seals to move brake fluid. In modern systems the theory hasn’t changed much, but master cylinders have become more complex. All modern master cylinders have dual pot or twin circuit systems where, if a leak develops in the front brakes, the rear brakes will function as normal and vice versa. There is usually an electronic sensor in the reservoir that sets off a warning light if brake fluid drops below a certain level. The two circuits are pressurized by a pushrod attached to the brake pedal. As you push the pedal, this pushrod moves one piston in each circuit, compressing the brake fluid and sending it to the calipers or wheel cylinders. Seals can wear out allowing leaks, springs internal to the master cylinder can rust and fail, and pistons can rust to their bores. Master cylinder bore size is matched to the size of the bore of the calipers and/or wheel cylinders. Any master cylinder that is not functioning properly should be replaced as soon as possible.
Vacuum brake booster or hydroboost
If your 4x4 has “power brakes,” it either has a vacuum-assist booster between the master cylinder and the firewall or a hydroboost booster. Power systems are usually only associated with braking systems that use disc brakes on the front wheels at the very least. With vacuum power brakes a vacuum booster uses vacuum from the vehicles engine to multiply the braking power when you press the pedal. It does this by opening a valve on one side of a diaphragm that is under vacuum from the engine. As you let off the brakes, this valve closes and vacuum is restored to both sides of the diaphragm. Hydroboost uses a similar mechanism but derives its assist from pressurized fluid from the power steering pump. Brake boosters fail when the diaphragms or one-way valves that help them function dry and crack necessitating replacement of the booster for new or one that has been repaired.
Proportioning valve, combination valve, or metering valve
Those are three names for the same part that helps regulate how much compressed fluid travels to the front or rear brake circuits. As you apply the brakes in any vehicle, the weight transfers forward. This makes the rear brakes of your 4x4 less effective as there is less weight holding the rear tires against the ground. Therefore, if you apply the same braking force to both the front and rear circuits the rear tires will lock up before the front. A piece of brass or aluminum is machined with various ports, pistons, and springs that regulate the amount of fluid that gets sent to the rear brakes. The aftermarket also sells a couple different types of proportioning valves that allow adjustment to the rear circuit. We’ve used these valves to “tune” our rear brakes action on- and off-road in many of our project vehicles. If a vehicle sits for too long, these valves can become locked up due to moisture trapped in the system. In that case, we've disassembled them, cleaned up the parts as well as possible, and reassembled using some brake fluid as a lubricant. If internal parts are too corroded, you may have to replace the part with an aftermarket version.
Using your vehicles computer and sensors that provide wheel movement feedback, the ABS pump uses a series of valves and an electronically-controlled brake fluid pump to add or subtract compressed fluid to right or left, front or rear brake circuits. Most of these systems used to only function when the driver applied the brakes, but new vehicles with stability control, an electronic stability program (ESP), or electronic stability control (ESC), will apply braking force to different wheels at different times if the computer has decided that the vehicle is traveling too fast for a corner or can detect that the vehicle is sliding. Off-road, these systems can work miracles or fail horribly as conditions can change rapidly and sometimes you want your brakes to lock up while off-road. A dedicated off-road rig may even have the ABS or stability control defeated or removed.
Hard brake lines
Hard brake lines are simply that, a sealed conduit to transfer compressed brake fluid from your 4x4’s master cylinder to the calipers or wheel cylinders. Most hard brake lines are made from steel, stainless steel, nickel/copper alloy, and occasionally plastic. Because most brake fluids attract water, any non-stainless steel brake lines can rust and develop holes from the inside out, and should be periodically visually inspected. This is also one of the main reasons to have your brake system periodically flushed to remove corrosion and moisture that could get trapped in the system. On a 4x4, hard brake lines can also get smashed by rocks on the trail or by modified parts hitting them.
Flexible brake lines
Flexible brake lines can be found between the master cylinder and proportioning valve, ABS pump, frame, or between hard-mounted hard brake lines on the frame and the vehicle’s axle, or a wheel caliper. A flexible brake line has a tough life as it must remain flexible while containing compressed brake fluid. Most flexible brake lines are made out of some combination of rubber and woven fiber or flexible metal mesh and rubber or plastic. Because these parts of the system move as the calipers apply braking force and as the suspension cycles they can dry, rot, crack, or balloon. All of these things will cause sub-optimal brake performance. Any flexible brake lines that show any signs of wear should be replaced immediately. Flexible brake lines can also get damaged when snagged on trail debris, or if stretched to the point of braking on modified 4x4s. It’s important to address brake line length or at least mounting position with any suspension lift or modification. Most aftermarket flexible brake lines are available as longer than stock and with stainless steel mesh to prevent swelling.
Brake wheel cylinders and calipers both have bleeder valves on them. These allow bleeding of the brake system, which means purging of trapped air or old brake fluid. “Cracking” your bleeder valve just means to open it up and let air or fluid out of the system. Bleeder valves have a barb for a hose to be temporarily attached so no air can leak back into the system when it is opened. Also, bleeder valves are almost always at the top of a caliper or wheel cylinder to keep air bubbles from being trapped. If you’ve just added new calipers and both bleeder valves are facing down, you will never get air purged from the system and you’ve put the right caliper on the left side and vice versa.
The Banjo bolt is a drilled bolt that allows brake fluid to move from the brake hose or brake line into the brake caliper.
Drum BrakesOnce the gold standard for stopping cars and trucks, drum brakes are all but extinct in modern 4x4s. There was a time when all four wheels of a 4x4 where slowed with drums. Full drum brake systems leave a lot to be desired when compared to more modern braking systems of the past 40 years. That’s roughly when factory engineers began to add disc brakes to front axles of 4x4s to improve stopping performance, but drums stayed in place on the rear axle, doing their job slowing our trucks for decades. Somewhere about 10 years ago rear drum brakes became all but obsolete on light-duty 4x4s. Disc brakes in general have fewer moving parts more closely adhering to the Keep It Simple Stupid (KISS) engineering rule. Despite this, drums are still found regularly out on the road and trail and with proper maintenance of components and proper adjustment drum brakes work just fine for most vehicles. One positive feature about drum brakes is that, because of their design, they have a way of passively assisting braking. As the brakes are applied and the pads get pushed into the drum, they tend to get sucked or pulled further in decreasing pedal effort. Since this does not occur with disc brakes, most disc brake systems have power assist to lessen the forces needed to stop.
Drum brakes are named for the bell-like metal drum that you see when the wheel is removed. The drum is a mechanical bell that turns with your 4x4’s axle and wheel. As the brakes are applied, the shoes are forced outward and into the inside circumference of the drum. Drums can wear to the point that they are too large for the shoes to work and they can get too hot and crack. On 4x4s, drums often also act as a trap for mud when submerged in muddy water, promoting rust and leading to premature failure.
Drum brake shoes are the fiber-based wear items that rub against the inside circumference of a drum brake. Generally, the fiber material is glued or riveted to metal that has provisions for pivot points, springs, cables, and retaining clips. Shoes generally have one end fixed (yet adjustable) on a pivot point, while the other end is attached to a piston coming out of the wheel cylinder. These pistons push the shoe against the drum as the brakes are applied.
Wheel cylinders are cylinders with a fitting and pistons. As compressed fluid is moved into a cylinder the pistons move out and away from the center of the piston. Wheel cylinders can wear out and are subject to seal failure as well as damage from rust. Wheel cylinders can be honed, have new seals added, and are easily rebuilt or replaced with new parts if they start to leak.
Disc BrakesRotor or disc
Once again, the name of this brake type is derived from one of its main components. The disc or rotor is pinched between two brake pads via a caliper as braking forces are applied. Rotors are a wear-point and can be worn out of specification ranges (worn too thin) but generally see little other failures. Rotors can warp from excessive heat and can have groves cut into them by oddly worn pads or foreign objects. As a point of friction, disc brake rotors can get very hot. Aftermarket performance rotors may be larger than stock and are often slotted, drilled, and finned to help deal with heat that ultimately can damage them.
Brake pads are the fiber-based friction wear components that interface with your 4x4s rotors. Composite fibers are riveted or glued to steel backings that help locate and hold them in the caliper. Pads are a wear-point, and different compounds can be tuned to change braking performance. Proper brake pad break-in is also important to longevity and performance of a disc brake system
The caliper of a disc brake system in its simplest form is a metal housing with fluid fittings and a piston with some structure to hold the brake pads. The piston floats on slide pins and as brakes are applied fluid causes the piston to force the pads onto the rotor. Piston size and number both play a role in brake performance. Most aftermarket brake calipers work with larger rotors and contain multiple pistons to help apply pressure to the brake pads and rotor.
Slide pins or slide mounts
Slide pins are fittings that allow the caliper to float or slide back and forth a given distance. Without some way for the caliper to move relative to the rotor, axle, and wheel a disc brake system doesn’t function. Slide pins should be lubricated but despite this they can become rusted and difficult to move. A brake system that is properly bled but requires pumping for the brakes to grab may have a caliper slide pin sticking issue.