Tech Article: Brake Fluid and Why Change it

by Ben Pila

There are many fluids that need to be changed every now and then. Brake fluid is important and needs to be changed when it gets dirty and as a preventive maintenence. A good interval to change the fluid is about every 30,000 miles or if it looks dirty or you are hard on brakes sooner.

Brake fluids are classified in three classes by the DOT (Department of Transportation.) Most of us use the DOT 3 fluid but Dot 4 offers a higher boiling point which is better for us who use our brakes hard on back roads. There is also a DOT 5 fluid which is also called Silicone brake fluid. This type of fluid is different from the DOT 3 or 4 in that is is made of silicones and doesn't absorb moisture (hygroscopic) like the other brake fluids do. It's the moisture and contaminents that break down the DOT 3 and 4 fluids which is the reason to change the fluid in the first place. If you don't change it, what happens is moisture enters the system via the vented fill caps on the brake master cylinder. This moisture along with dust, mixes and travels in the system eventually making it's way to the lowest points in the system. The lowest points would be the brake calipers and the rear wheel cylinders (for those that have rear drums.) If the moisture sits against the aluminum and chrome parts in the system, it corrodes until the seals of the wheel cylinder or caliper pistons get damaged then fluid begins to leak out and you know the rest. Also as the fluid get contaminated, it becomes diluted and the boiling point of the fluid gets lower.

This may not be a problem just around town but if you've ever driven down a mountain road like to Borrego Springs, you know how important the brakes are. It's when you use the brakes very hard, repeatedly that the boiling point may be a factor. DOT 3 fluid has a minimum wet boiling point of 284 degrees F. This is only if conditions are good and brake fluid is clean. The DOT 4 fluid has a higher boiling point and is an improvement over the DOT 3 for that reason. I've used the Castrol LMA DOT 4 and it's worked ok.

Since I tend to have an agressive driving style on the back roads, I've boiled this fluid and have since changed to the ATE Super Blue Racing fluid. This fluid is very reasonable in price and has a wet boiling point of 392 degrees F. I've tried hard to boil this fluid but it remained stable. My front pads just overheated and faded.

What happens when the brake fluid boils? It turns to a vapor and then your pedal goes to the floor and you have no brakes. Quite scary on a mountain road. My recommendation is to check you brake fluid by removing the fill caps on the reservoir and if you see a murkey, "muck" like dark fluid, then it's time to change. Once you change it, then you can stick to a regular schedule. Follow my steps below to change the fluid.

Changing your brake fluid

1. Remove the fill caps off the brake master cylinder.

2. Using a turkey baster (.99 at any .99 store,) suck out the old fluid and put it into an old jar with a lid. Be careful not to get any on the paint as it works like paint stripper. Used water to neutralize any spills.

3. Remove the plastic filter screens from within the reservoirs and suck out the remaining fluid from the reservoir.

4. Take the plastic filter screens to a sink and rinse them with water and use your fingers to rub off any dirt as you rinse them in water. Shake them dry and set them aside on a paper towel to dry.

5. Using clean paper towels or shop towels (lint free rags,) wipe out the inside of the reservoirs clean of any dirt etc. If you want an extra clean job, you could use a 10mm wrench or socket and physically remove the plastic reservoirs off the master cylinder (note which one goes where if they are different.)

6. With the reservoirs and plastic screens clean, reassemble and top off with your favorite brake fluid. Next comes flushing the remaining fluid in the lines and brake parts. It may help to have the car on jack stands and wheels off on some models that have the bleeder screws mounted high and hard to reach.

7. Beginning with the right rear wheel's bleeder screw (usually located on back of the drum or caliper,) take a 10mm or 8mm wrench and break the fitting loose first then attach a 3 foot piece of 3/16" clear polyethelene tubing to the bleeder valve nipple and put the other end into a clear container that has a lid. Try to set the container so that the tubing is higher than the bleeder fitting. This way any air will rise and not flow back into the system.

8. Open the bleeder about a turn then begin to pump the brake pedal in about 1" strokes or just enough to move fluid out into the tubing.. Reason you don't want to do full strokes to the floor is if the brake master is old and corrosion has started, you could possibly damage the piston seals of the master cylinder by the seals ripping over the corroded areas and ruin the master cylinder. So just pump the pedal enough to move fluid through the tubing. Continue pumping until the fluid comes out mostly clear (or blue if you use the fluid I do.) Pay attention to the brake fluid level in the reservoir as it will get low as you pump. Top is off before it gets too low or you'll have to bleed the master cylinder to get the air out.

9. Once the fluid comes out mostly clean, close off that bleeder and move over to the left rear wheel and continue the same process. Then do the right front and finally the left front.

10. Top off the brake reservoir, wipe down the caps (use some brake parts cleaner spray if needed,) then install the caps and wash off any spilled fluid with water. Your done!