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 pressurised tank that forces fluid in at the master cylinder so that it can be removed at the bleed ports more quickly.
Sometimes, it can also be done with a combination of means, such as when a master cylinder is pressurised 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.
The Cost of a Brake Fluid Change
How much to change brake fluid is pretty straightforward and is usually a set price regardless of the vehicle. In some cases, a vehicle with a large reservoir – such as some ¾ ton trucks – costs a little more as it takes more fluid.
The general brake fluid flush cost ranges from £45 to £120.
Prices are usually about the same for all the methods, although a shop on the higher end of the price scale is more likely to have fancier equipment and get the job done faster. The result is ideally 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, sealing the fluid away from the atmosphere isn’t practical.
In the master cylinder brake fluid reservoir, the level of fluid steadily drops as brake pads wear and more is taken up in the callipers. Also, each time the brakes are applied, the level of the fluid goes down a little, then back up when they are released.
The master cylinder caps are vented, so this fluid displacement can’t result in a pressure or vacuum in the reservoir, which would cause the system to draw air in past the seals elsewhere. This vent, along with the regular cycling of fluid level, 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. 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, which 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 the owner’s manual recommends for the vehicle, that’s 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.
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 being silicon-based, so it can only be used in a completely evacuated system or on a newly-built system.
It has some advantages in not being hygroscopic, but 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. If any problems are present in the brake system, they would likely be found at that time, and it’s not uncommon to have a recommendation for work to be done 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.
If a vehicle is brought in just for service, especially if it’s brought in at a place that regularly services the vehicle, it would be normal for the shop to go over the service records and recommend that any other services due be done at the same time.