The basic goal of the service is the same on all vehicles – to replace the old fluid in the braking system with new fluid. This can be done by gravity bleeding, with a vacuum applied at the bleed ports, or with a pressurized tank that forces fluid into the master cylinder so that it can be removed at the bleed ports more quickly.
Sometimes, it can also be performed with a combination of means, such as where a master cylinder is pressurized and a vacuum is used at the bleeders to make the job even faster.
There are various time and mileage recommendations from the various car manufacturers. The general range of mileage recommendations is 20,000 to 50,000 miles, and time recommendations mostly range around the three-year mark. That information should be in a vehicle’s owner’s manual and maintenance packet.
The Cost of a Brake Fluid Change
How much it costs to change brake fluid is pretty straightforward and usually a set price in most shops regardless of the vehicle. In some cases, a vehicle with a large brake fluid reservoir, such as some ¾ ton trucks, costs a little more as it takes more fluid.
The general brake fluid flush cost range is from $80 to $150.
Prices are usually about the same for all the different ways of doing the job, although a shop on the higher end of the price scale is more likely to have fancier equipment that gets the job done faster. The result should be about the same in all cases.
Why a Brake Fluid Flush is Recommended
Almost all automobiles use either DOT 3 or DOT 4 brake fluid in their braking systems. Both are polyglycol ether fluids, which are inherently hygroscopic, which is to say that they absorb moisture from the atmosphere. Because of the way that braking systems work, completely sealing the fluid away from the atmosphere isn’t practical.
The level of brake fluid in the master cylinder reservoir steadily drops as brake pads wear and more fluid is taken up in the calipers. Also, each time the brakes are applied, the level of the fluid goes down a little, then back up when they are released.
Master cylinder caps are generally vented, so this fluid displacement can’t result in a pressure or vacuum in the reservoir, which would incline the system to draw air in past the seals elsewhere. This vent, along with the regular cycling of fluid levels, results in the air circulating within the master cylinder reservoir, where the brake fluid absorbs moisture from it.
One of the most important qualities of brake fluid is its high boiling point, and its effective boiling point drops quickly as it absorbs moisture. A great deal of heat is generated when braking, and some of this heat will transfer through brake components into the fluid.
If the brake fluid boils, the water in the fluid becomes a gas that compresses, with the effect that the brakes no longer work. That’s called “brake fade”. It happens pretty rarely, but is well worth the maintenance to avoid.
Excess moisture in the brake fluid can also cause corrosion in the brake components; this is also a situation well worth avoiding.
Types of Brake Fluid
The main types are DOT 3 and DOT 4. The boiling point of the first is 401 degrees, and the boiling point of the second is 446 degrees. This is for brand new fluid from a sealed container.
The brake fluid replacement costs for both are about the same. As a rule, whatever fluid is specified by the owner’s manual or the brake fluid cap is the fluid that should be used.
As far as compatibility, they can be mixed, but it’s usually not done unless the correct fluid is unavailable. If a manufacturer specifies DOT 4, for instance, but DOT 3 is the only fluid available, then DOT 3 can be used. But this lowers the boiling point below what the manufacturer required, and so it should be changed back out for DOT 4 at the soonest convenient point. There are some DOT 3-4 fluids that can be used safely in either system.
DOT 5 is the third type of brake fluid, which is generally not used in automobiles. It is incompatible with DOT 3 and DOT 4 so it can only be used in a completely evacuated system or on a newly-built system.
DOT 5 has some advantages in that it is not hygroscopic, but it has a disadvantage in that it can’t be used on a vehicle with an ABS system. The cycling of valves that operate the ABS can cause DOT 5 to cavitate and aerate, at which point the brakes don’t function.
Other Things That Might Be Recommended
Flushing the brake fluid involves working on all four corners of the brakes as well as the master cylinder. It’s normal for the brake system to be given an overall inspection during the service, and for any work that might be needed to be priced out and recommended at the same time.
The brake flush is part of a vehicle’s basic maintenance. Usually, this is done at the time of a 30,000 or 50,000-mile service, along with other routine maintenance on the list. It would be normal for the shop to go over the service records and double check for any other services that might be due at the same time.
Frequently Asked Questions
Moisture buildup in the brake fluid is the main problem, which can cause corrosion and wear in the system, as well as the risk of brake fade (loss of brakes) in high heat situations.
Most shops use a pressure bleeder for the job, as that is the fastest way, but it is possible to gravity bleed a brake system. That involves filling the master cylinder and leaving the cap loose, and opening the bleeder valves on the calipers or wheel cylinders one at a time. Brake fluid will drip out of the bleeder valves, and the master is topped off as necessary. Letting each one lose a half cup or so would change out a quart of brake fluid, which is about what a normal service does.
One way is to look at the brake fluid or shine a light through the reservoir; the fluid should be clear. If it’s dark, it’s probably due for maintenance. Brake fluid test strips that measure the amount of moisture in the fluid are also available at parts stores.