Changing Brake Fluid and Brake Hoses in Bentley and other High End Cars

All high end carmakers recommend replacement of brake fluid. The only thing that varies is the interval, which ranges from one to three years on most brands. The reason for the change is that DOT3 and DOT4 brake fluids are hydroscopic; that is, they absorb water. The water causes rusting of brake components from inside, which causes caliper failure. Fluid with moisture in it also has a lower boiling point, and cars that are run hard can suffer sudden brake failure if the fluid boils on hard braking. Finally, the fluid picks up particles of rubber from seals and hoses and that can contribute to sludge formation that clogs critical components if left in place.

In addition to the manufacturer recommendations car clubs and race tracks often have their own rules for brake fluid flushing before track events. At Robison Service we recommend following the manufacturer or track change recommendations with one caveat: If the car lives in a humid area we recommend checking moisture content periodically with a test strip and changing early if it becomes excessive.

Carmakers also recommend periodic replacement of the brake lines. Your brake lines flex with every movement of the tires, and they are subject to deterioration from oxidation (dry rot) just like all the other rubber bits on your car.

Most carmakers recommend brake hose changes at intervals ranging from 6 to 10 years. Land Rover and BMW have both had recall when they began seeing hose failures in the field. Brake hoses are critical parts that are not easy to evaluate externally and they are critical to your safety so I recommend they be changed by the schedule, and certainly when ten years old.

In some cases the carmakers also want us to replace the steel pipes adjacent to the front brake hoses because they are stressed from the combination of steering and the suspension’s up and down movement. Bentley is an example of a carmaker who does that. Here is an illustration of a hose update on a Bentley GT. Other high end cars will be similar.

In this photo you can see the front hose assembly on the car:

Here is the rubber hose and steel pipe, removed from the vehicle.

Bentley wants us to replace the screws and retaining clips as well, as you see here:

In the back of the Bentley GT there are 6 rubber hoses, all of which need to be changed:

Once the hoses are changed the brake system has to be thoroughly bled. You can get a jump start on this by using a pressure bleed system and letting fluid flow through to each wheel, but you will probably need a factory test system for the final phase to activate the ABS/DSC pump and bleed that. Older cars won’t have that but any high end vehicle from 2000-onward will.

On some cars, hose replacement is limited to four hoses. The Porsche 911 is an example of that, and the job can be done in a few hours. Afterward the fluid flushing takes up another hour. Cars like the Bentley GT require replacement of some steel pipes in addition to the hoses, and on the Bentley there are 6 hoses, not 4. Cars like the Bentley will be most of a day's work to service.

The most time consuming cars are the older Rolls-Royce and Bentley which have unique braking systems with 20 or more hoses. Those cars can be 20-30 hours of labor to update hoses and flush fluids.

On older cars don't be surprised if you have to replace some of the steel lines. If they are old and rusty they may break when the hoses are unscrewed. The car in the photo below needed new steel lines in addition to the rubber hoses:

It’s important that you use the correct brake fluid. DOT3 is the fluid used in most cars through the 1980s. At that point DOT4 came along as a higher performance fluid, and we use that in place of DOT3 almost everywhere today.

DOT4 is the standard fluid in most current cars.

DOT5 is a silicone synthetic fluid that is popular in some antique car circles because it won’t damage paint. We have seen problems when DOT3/4 systems were changed to DOT5 and experienced failures, so we discourage that practice now.

Some cars need lower viscosity brake fluids for high performance stability control, and for those vehicles we use variants of the traditional DOT4 fluid.

When you change hoses you should always inspect the pads and rotors. With most cars the rule of thumb is that the friction material must be at least as thick as the metal brake backing plate. In the car below (a Bentley GT) the pads are about twice as thick as the backing plate.

Another think I suggest is that you make sure the pads are free in the caliper, and all the sliding pieces are free and greased if necessary. Note that some cars use lube on the calipers and others don't.

John Elder Robison is the general manager of J E Robison Service Company, celebrating 30 years of independent Bentley, Rolls-Royce, BMW/MINI, Mercedes, and Land Rover restoration and repair in Springfield, Massachusetts. John is a longtime technical consultant to the car clubs, and he’s owned and restored many fine British and German motorcars. Find him online at www.robisonservice.com or in the real world at 413-785-1665

Reading this article will make you smarter, especially when it comes to car stuff. So it's good for you. But don't take that too far - printing and eating it will probably make you sick.