Hydraulic brake lines and hoses aren’t something most people think about when it comes to vehicle maintenance. That’s understandable, considering automakers don’t list periodic replacement of them anywhere under routine services. However, many factory-spec guides state to check the condition of brake lines for any issues, and advise accordingly. This may leave you wondering when, or if, brake lines ever need attention.
Brake lines do require attention. Because they play a huge role in the operation and safety of your entire brake system, checking their condition periodically is important. Ultimately, it’s their age and condition which determines when replacement is needed. In this article, we’ll describe which sections of the lines need replacement most often, how they wear, and how to spot signs of trouble during inspection.
About Brake Lines
First, picture a master cylinder unit pressurizing brake fluid when you step on the pedal. Then picture that pressurized fluid being pushed through a series of rigid metal tubes and flexible lines made of rubber or braided stainless steel - until it reaches the brakes at all four wheels. Without those tubes and lines, commands sent by the master brake cylinder would never reach the wheels, and your vehicle would have no ability to stop whatsoever.
When using the expression “brake lines”, some clarification of parts is helpful. First, the rigid, metal tubes we mentioned are narrow, thick-walled, and heavy-duty in nature. These start right at the source (master cylinder), then run down along the underside of the vehicle. This tubing usually doesn’t need replacement, and don’t require your concern unless they become damaged or rusted.
Because these tubes offer zero flex near wheels which move up and down with the suspension, sections of flexible brake lines are installed to make the final connection to the brake calipers. Short in length, these “lines” are made of rubber – and are the pieces of the system that do need inspection and eventual replacement. So when you’re looking for brake lines, what you want are the flexible end sections only. Specially designed fasteners at hose ends provide reliable, leak-free connections.
For the purposes of this article, we’ll use the term “brake lines” when making general references to flexible lines. As a note, all types of brake lines sold in the United States must conform to a series of uniform standards set by the U.S. Department of Transportation (DOT) when it comes to construction, pressure ratings, labeling, and other criteria.
Rubber Brake Lines
Most factory vehicles are built with rubber lines consisting of a fluid-resistant inner liner surrounded by several thick layers of protective rubber. These are great for flexibility and easy routing through tight spaces. For decades, rubber compounds have proven themselves to be resilient in tough conditions and durable as time stretches on.
For extra protection in certain spots, protective metal or PVC sleeves may be installed around rubber hoses when there’s a risk of direct contact with hot engine or transmission components.
Stainless Steel Braided Lines
Stainless steel brake lines feature a sealed inner hose typically constructed from flexible Teflon surrounded by Kevlar and other protective layers in addition to a woven tapestry of braided steel strands (for strength). While these cost more than rubber brake lines, they are much stronger and offer a higher level of protection against flying rocks and other foreign debris found on roadways.
Stainless steel brake lines also swell a lot less than rubber ones do when brakes are applied and fluid pressure builds. For this reason, changes in fluid pressure reach the wheels faster and more efficiently – leaving the driver with a responsive-feeling brake pedal. However, the amount of actual fluid force (torque) reaching the wheels is not affected by the type of brake line, so braking strength will be the same in either case.
"DOT Compliant" vs. "DOT Approved"
Shopping for replacement brake lines, you'll typically see manufacturers state their products as "DOT compliant". That means they've met all minimum standards outlined in government Department of Transportation regulations. This is good.
The phrase "DOT approved" might sound like the Department of Transportation actually tested the product themselves and put a big red "approved" stamp on it, but that's not the case. Testing is done by other agencies, not the DOT. So in the end, DOT approved really means nothing more than DOT compliant does.
How Brake Lines & Hoses Wear
Brake lines do develop issues over time, and should be replaced for any of the following reasons.
They Degrade From Within
If the metal brake tubes have developed significant rust, that's a big reason to replace them. All brake fluid will absorb water over time; that is a principal reason to flush the brake fluid on a regular basis. If that is not done, as water builds up in the fluid, it can start to cause rust so that the metal brake lines corrodes from within. You may see no sign of this rusting until a hole breaks through the surface, at which point you run the risk of losing brake pressure.
With flexible rubber hoses, the rubber itself slowly starts to degrade from within because of heat and moisture in the brake fluid. In a worst-case scenario, that heat and moisture can cause rubber to lose its structural strength in spots - leading to a line that collapses or breaks apart. If the fluid-resistant inner lining should crack, brake fluid then works its way up through the outer layers of rubber where it may cause visible bulges or cracks.
Brake hoses can degrade from within as discussed above, eventually leading to a complete breakage and attendant fluid leakage. The rubber hoses are also exposed to the elements, and therefore, like any other rubber product, are subject to degradation from UV rays, salt, road debris, and the like. If neglected, the rubber can eventually fail and cause the loss of brake fluid. The saving grace is that all cars since the late 1960s have dual brake systems, so that in the event of a system failure, you should have at least two working brakes, although the pedal may have little resistance.
Brake lines that have collapsed in one area means that particular line is effectively pinched and shut off. As a result, brake fluid which is supposed to flow to that specific wheel cannot make it there. Reduced fluid pressure at the caliper means reduced braking action, and the uneven levels of grip from left to right will cause the vehicle to pull to the side.
Rubber brake lines can collapse internally as rubber becomes aged and deteriorates from within. Unfortunately, this makes the condition hard to spot on the outside of a rubber hose. Stainless steel braided lines typically don't collapse from the inside, which greatly reduces the chances of the problem occurring. However, they can become permanently dented or bent by flying road debris that might bounce right off a rubber hose. Upon inspection, you'll be able to spot any deformations in a metal hose easily.
Signs Of Trouble
Warning signs that a brake line has already become compromised include fluid leakage, an illuminated "Brake" system warning light, and corrosion of metal. Other than feeling wonky brake action when you step on the pedal, these will be easiest signs to see.
When rubber lines reach the point where they're degrading internally, they often begin to seep or leak visibly as inner layers of rubber weaken and give up their ability to prevent fluid from squeezing its way out. Rubber lines will typically leak before steel ones do. However, steel ones can eventually suffer the same fate when their inner brake tube materials grow old. Leaks that are more severe signify a hose could possibly give out soon due to cracks. Dripping brake fluid can also damage brake sensors or wheel speed sensors if it comes in contact with them.
"Brake" Warning Light Comes On
"Brake" system warning lights come on when sensors built into the brake pads indicate a minimum thickness point has been reached, or when brake fluid levels in the reservoir have dropped below a safe point. A leaking or cracked brake line will cause the fluid level to drop noticeably.
Spongy Brake Pedal Feel
Another way to tell you've got a fluid leak is if the brake pedal feels spongy or squishy when you step on it. If you're noticing this, then air has leaked into your brake system in place of the fluid that's leaked out. As pockets of air bubbles are compressed and squeezed around by moving brake fluid, a soft and mushy pedal results.
As with any brake system repairs that require removing a fluid line, the fluid in the system must be completely flushed out and replaced with new fluid after the repair is completed. This ensures any air in the system gets pushed out, along with water moisture that normally collects. For more details on this process, see our related article When Is It Time To Replace My Brake Fluid?
Checking Brake Fluid Lines
When making a closeup inspection of rubber brake fluid lines, first check for the presence of small-yet-visible pin holes or significant cracks. Either of these can lead to a rupture. If there's no leakage, check for chafing or abnormal bulges in the rubber. A blister on the outer surface of the line means fluid has escaped from a compromised center tube and is lurking underneath the skin of the outer layer, which could blow any time.
Check to ensure connections are tight, especially if any clamps are used. Since the U.S. Department of Transportation requires two parallel lines be printed into the surface of the rubber, these can be used to see if the line has twisted in place because its mounting points may be weak, it's being bumped by a moving suspension component, or it's got too much slack.
Once the engine has been off for awhile and things have cooled down, gently squeeze flexible line rubber sections by hand. They should feel firm and strong - not stiff, soft, or brittle. A line that feels soft is weak because the rubber has decayed from within, while rubber that's hardened from age can easily crack and rupture under pressure.
If you've got a second person to help you, take your inspection a step further. Have them start the engine and pump the brakes to simulate pressures that are present during driving. A hose that's in good condition will retain its size and shape. If you see any signs of swelling or shape-shifting, it's time to replace the line. In fact, it's time to replace all of your lines. As we mentioned earlier, look for damage to any braided steel lines - which be easier to see because the metal will clearly retain any signs of trauma.
And don't forget to inspect the rigid metal tubes running from the flex lines up to the master cylinder. They can become damaged too. In less-than-common scenarios, the metal tubes can suffer from rust in areas of high humidity - especially if a vehicle is regularly parked over unpaved ground. If there are any sign of corrosion or damage, those tubes must be replaced.
Replace All Of Your Flexible Brake Lines Together
We'd like to stress the importance of changing all of your flexible brake lines at the same time when there's a problem - unless an unaffected one is relatively new itself. If that's the case, make sure others you install now are an exact match when it comes to diameter, size, and fluid flow.
Since all of a vehicle's flexible brake hoses see the same use and abuse, they wear at the same rate. So if one has gone bad, the others aren't far behind.
Opportune Times To Replace Brake Fluid Lines
An opportune time to replace brake lines is virtually any time the car is off the ground with wheels off. Since the brake system must be bled of air whether one or four brake lines are changed, it makes good economic sense to replace all four at the same time. Doing one at a time means paying repeatedly for labor operations such as getting the car on the lift, taking the wheels off, bleeding the brakes afterward, etc.
Likewise, a good time to replace brake fluid lines that are just starting to leak is at the same time any routine brake fluid flush service is due. Other brake system repairs that require a full fluid flush afterward are replacement of the master cylinder, calipers, metal tubes, or various anti-lock system hardware.
Another easy opportunity to slip a new set of flexible brake lines in place is whenever the calipers are unbolted from their mounting brackets. This is normally done during brake pad or rotor replacement, or when brake rotors are removed to work on items behind them such as wheel hubs, bearings, axle shaft seals, or the axles themselves.
Our article Special Tools Used In Brake Service will give you an idea what equipment is needed to perform the jobs we've mentioned about. For more details on bleeding out the brake system and putting new fluid in, read our article on performing a brake fluid flush.
To find the metal brake lines and rubber brake hoses for your vehicle, just enter its year, make, and model in our "Select Vehicle" box. And if you have any further questions, we've got knowledgeable reps that will be glad to help you seven days a week.