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4x4 Brake System Basics

Leaf Spring
Christian Lee | Writer
Posted October 1, 2011

Fix it Before it Fails

Most of us only think about our vehicles’ braking systems when there’s a problem: weird noises, stiff or spongy pedal, right or left pull, or even complete brake failure. All of these situations can be avoided, however, with some basic annual brake maintenance.

Simply put, look at your brake system. Check line connections for leaks and inspect lines that may be susceptible to abrasion or being pinched or dented, particularly the hard lines along the axles and the braided or rubber lines. Rubber lines are inferior in our opinion and are best off being replaced with braided steel lines; they are much stronger and don’t suffer the same bulging and cracking effects as rubber lines. Other likely leak points are the brake calipers in disc brake systems and wheel cylinders in drum systems.

Whether disc or drum or both, the braking surfaces should be inspected for irregular wear and damage. Brake drums should be measured using a drum gauge to determine whether the inside diameter is still acceptable. Most drums have a maximum diameter or machining spec stamped or cast in to the face of the drum. Brake pads should also be inspected and replaced as needed.

Just like engine oil and transmission fluid brake fluid needs to be changed periodically. Over time steel brake lines allow air and moisture into brake fluid. Moisture in brake fluid is not good. It causes internal corrosion and rust that can lead to brake line/system failures. The most common brake fluid is DOT 3, which is glycol-based. Mineral/glycol-based brake fluid can actually absorb condensation and contain as much as 2-percent water after just one year. Glycol-based brake fluid also absorbs moisture naturally from the air. An open container of brake fluid has a shelf life of about a year.

The Department of Transportation (DOT) number indicates the brake-fluid boiling point. Slam on the brakes or brake hard continuously during a typical trail run and brake fluid temperature rises significantly. DOT 3 means the brake-fluid has trouble when there’s 3-percent water in the fluid. This is known as the fluid’s minimum boiling point. For each percent of moisture absorbed, the fluid’s boiling point drops 50 degrees. That means brake fluid with a lot of moisture will boil when you brake hard, creating air pockets and a spongy pedal and likely other side effects. DOT 4 brake fluid is also glycol-based and has a minimum boiling point of 446 degrees F. It can be mixed with DOT 3 fluid to improve fluid properties and raise the boiling point, but we suggest completely flushing and bleeding the system with DOT 4 if you take this route.

Brake Pads
Brake pads are brake pads, yeah? That’s why we usually leave the parts store with the cheapest or one step up from the cheapest brake pads that will fit our application. None have led to brake failure to this point so they must be good, right? Yes, correct, but that doesn’t mean there isn’t something out there capable of performing better. There are a number of performance type brake pads on the market that can actually add some improvement to your brakes. High-friction brake pads can and do help braking quality, especially when towing or being used aggressively, such as during a weekend trail run. You’ll pay more for what you get but you’ll also likely reap the benefits as well.

Bleeding Techniques
Hydraulic brakes don’t like air. There are several approaches to brake bleeding, the most conventional involving a pickle jar, a vacuum hose, a buddy with a good foot, and a brake-bleeder wrench. When you don’t have a friend to help with the bleeding, you can use the gravity approach, which is to position jars and hoses at all four brakes, fill the master cylinder, and open all bleeders. The system should purge itself of all air over several hours. It’ll work, but having a friend on hand is much faster. There are also a number of power bleeders available. Power bleeders create a vacuum at the brake bleeder. You basically just fill the master cylinder and open the bleeder, and the vacuum draws fluid and air out of the system and into a container in seconds.

Airplane Origins
Hydraulic brakes were invented in 1918 by Malcolm Loughead, one of the original founders of aerospace giant Lockheed-Martin. They have been in regular use on automobiles since the 1920s. Mechanical brakes cost less to produce and were less complicated to manufacture, but they weren’t as safe. As automobiles became faster and more mainstream, more and more were fitted with hydraulic brakes.


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