Thursday, July 8, 2010

Everything you ever wanted to know about Brake Fluid

How exciting!

Brake fluid is categorized by the DOT rating. Most new cars will be equipped with DOT 3 brake fluid. Your germans and supercars may use DOT 4, and very few will ever use DOT 5 will come in motorcycles and is a silicone based product.

Chemically speaking, the off the shelf brake fluids, like DOT 3 are based on glycol and glycol esters. DOT 4 is similar in composition except it contains borate esters. New to the brake fluid world is the DOT 5.1 that is made of of borate esters.

Do you know what real job of this fluid is? Brake fluid transmits pressure from the brake pedal to the ends of the brake system, where the calipers or drums are. This force from the fluid is applied to the pads (or shoes) against the rotation of the brake disc/drum.

Its the fluids rating or strength not to compress that allows it to do its job well.

Back to the Mr Wizard lesson. Did you know that H2O is not listed as an ingredient of brake fluid?


Water boils at 212 around degrees F. When braking, the rotors can and get HOT...over 1000 degrees of heat and transfer 400 of those guys or more to the caliper. Agua is not a good fluid to choose.

Unfortunately, Brake Fluid is hygroscoptic. DOT3 and DOT 4 brake fluids attract water. Right out of the air. Just like the coolant you use is probably glycol based just because it mixes well with water.

So, what do the numbers mean?

The DOT Numbers are the boiling point ranges that they achieve both dry (no water absorbed) and wet (about 3-4% water content). DOT 3 the dry boiling point is at least 401 and the wet 284 degrees. DOT 4 steps its game up to 446 and 311 respectively. Remember, that these are DOT minimums, and there are high performance brake fluids that exceed those ratings.

Some of the high performance fluids are rated well over 500 (to almost 600) degrees boiling point dry. Wet boiling points will also vary, but be well over 400 degrees.

The higher boiling points are critical to those of you that like to get your roll on at a race track or autocrossing. Once you boil your brake fluid, its done. You'll need more fluid. Sorry atari.

Most non racing folks will that their brake performance is just fine by sticking with the fluid that the manufacturer sold the care with, unless they use they hoon the vehicle to the extreme, such as a Autocross, Track, or if you live in the mountains and stuff. How do I know when this happens? You'll know because the brake pedal will start to feel like mashed potatoes as the fluid boils and the pedal may even fall to the floor.

Most joe blows never flush and replace their brake fluid. In fact, if you go to a shop and have a “brake job” done it is highly unlikely that they will do much more than bleed the brake lines. This means that the old – wet – clapped out – brake fluid will still be there. The water can react to oxidize brake components from the inside out. I recommend once a year, at least have a look at it. If its nasty, flush and replace with fresh brake fluid.

If you want to DIY, buy a Motive Bleeder or call a friend over, this can be done at home. If you cant change your own engine oil, go pay someone else to do this.

Unless your factory manual specifies something different you start with the right (passenger side) rear, then move to the left (drivers side) rear, then the right front, and finally the left front.

If you dont have a bleeder, you will have to work in concert with your assistant at the brake pedal. The goal is not to get any air introduced into the system while you bleed it. To prevent air from entering you need to keep the reservoir from going empty. So check it often and add new fluid as needed. If you do have a bleeder, keep the pressure up.

You should also remove as much of the old fluid from the reservoir before you add new fluid to the reservior. A turkey baster or similar device will allow you to do that. Just have spray “brake clean” ready in case any fluid get on painted surfaces. If left on paint it will remove it. PROTECT YOUR EYES.

You will need to continue this until you get nice, clean, new fluid out to the particular brake you are working on and then repeat for the other three. Be sure to keep enough fluid in the reservoir to prevent air from entering the system from that end.

Brake fluids gain contamination as they age and should be replace periodically. If you race you should change your brake fluid as often as you change your oil.