Automotive Brake Fluid


By Jeff Lucius


The Haynes Automotive Reference Manual defines brake fluid as "A compounded liquid for use in hydraulic brake systems, which must meet exacting conditions (impervious to heat, freezing, thickening, bubbling, etc.)." Well being impervious to heat is certainly the goal! The car's brakes convert the kinetic energy of the car into thermal energy, or more simply, they heat up as they slow the car down. The heat (energy) generated by the brake pads and rotors heats up the brake fluid in the calipers. If the fluid heats up enough, it will boil creating vapor in the system. Vapor in the hydraulic system (whether it is from brake fluid, water dissolved in the fluid, or air) is bad because it is compressible, reducing braking efficiency, possibly to the point of loosing the brakes entirely.

WARNING. Brake fluid is poison. Keep it away from skin and eyes. Do not allow brake fluid to contact painted surfaces.

The DOT brake fluid classifications (49CFR571.116) include a set of minimum specifications that are guidelines for manufacturers as to how impervious their fluid is to heat. The dry boiling point (when the fluid is fresh and contains no water) is the temperature at which the fluid turns to vapor. The wet boiling point (measured when the brake fluid contains 3.7% water) is related to how easily the brake fluid will absorb water (or how hydroscopic the fluid is). The lower the wet boiling point, the more water the fluid will absorb. Absorbing water is bad, so a higher wet boiling temperature means better brake fluid. And, of course, a higher dry boiling point is good too.

The table below summarizes the DOT guidelines plus the relative advantages and disadvantages of each fluid type. The actual performance may exceed the DOT guidelines and should be printed on the container. For example, Pyroil Premium DOT 3 Brake Fluid states a minimum wet boiling point of 291ºF. DOT 3 and DOT 4 are polyglycol-based fluids and can be mixed with each other. DOT 5.1 can also be mixed with DOT 3 and DOT 4, even though it is based on a different chemical and has about half the viscosity. If DOT 5.1 is specified for an ABS system, do not add or use any other fluid type. DOT 5 is silicone-based (not less than 70% by weight of a diorgano polysiloxane) and must not be mixed with or contaminated by DOT 3, DOT 4, or DOT5.1.

Standard Brake Fluids
  DOT 3 DOT 4 DOT 5 DOT 5.1
Dry BP ºF (ºC) 401 (205) 446 (230) 500 (260) 500 (260)
Wet BP ºF (ºC) 284 (140) 311 (155) 356 (180) 356 (180)
Kin. Viscosity
mm2/second		 1500 max @ -40ºF
1.5 min @ 212ºF		 1800 max @ -40ºF
1.5 min @ 212ºF		 900 max @ -40ºF
1.5 min @ 212ºF		 900 max @ -40ºF
1.5 min @ 212ºF
Advantages - Inexpensive
- Readily available		 - Available at auto parts stores
- Less hydroscopic than DOT 3		 - Does not attack paint
- Does not absorb water		 - Superior performance
Disadvantages - Attacks paint!
- Most hydroscopic		 - Attacks paint!
- More expensive than DOT 3		 - DOES NOT mix with DOT 3, DOT 4, DOT 5.1
- Water may "puddle" in the system
- Careful bleeding required
- Slightly compressible near its boiling pt.
- Very expensive, hard to find		 - Attacks paint!
- Expensive, hard to find

Probably the best thing you can do to maximize the performance of your brake fluid (any type) is to change it regularly (at least once a year) to minimize the amount of water in the system. According to the Car Care Council and SAE field tests, brake fluid can become contaminated by water (2% on average) within 1 to 2 years, and in substantially less time in humid climates. Brake fluid (other than DOT 5) readily absorbs water from the air in the master cylinder. This cannot be prevented and changing (flushing) the brake fluid is the only solution. Alternating the use of different-colored fluids can aid in knowing when the flushing process is complete. The dry boiling point of DOT 4 is higher than the wet boiling point of the best brake fluid (Castrol SRF excepted). Speed Bleeder Products makes a replacement bleeder screw for the brake calipers that allows one-person bleeding. Speed Bleeders cost $7.00 each; you'll need 4 plus one for the clutch release cylinder if desired.

While most brake systems will perform as designed using standard DOT 4, you may want to consider one of the popular high-performance brake fluids, especially for racing situations. Some of these are Ate's (Alfred Teves Engineering's) Super Blue Racing and "Super Gold" (same fluid but different color; also called Ate Typ 200), Motul's 5.1 and Racing 600, AP Racing's 550 and Ultra 5.1, and NEO's "Super DOT".

raceshopper.com sells Ate Typ 200 (or Super Blue Racing) for $9.95/liter, and other brake fluids at some of the best prices you will find. Motul Racing 600 can be purchased at Porterfield for $18/liter. Other retailers sell AP Ultra 5.1 for about $20/liter and AP 550 for about $28/liter. Brake Man sells their racing brake fluid for about $25/liter. NEO "Super DOT" sells for about $12 per 12-oz can ($34/liter). Castrol SRF has superior properties to the other brands listed but at about $65/liter (raceshopper.com) seems to me to be a bit expensive. Ford Heavy Duty brake fluid is cheap ($6/liter) and has a very-high dry boiling point, making it a favorite for the weekend racer that changes brake fluid frequently. However, the DOT 3-rated wet boiling point makes Ford Heavy Duty less than desirable for street usage. Do not mix any of these fluids, some of which are DOT 4 and others are DOT 5.1, with DOT 5 silicone-based systems.

High-Performance Brake Fluids
  Motul Ultra 5.1 AP Ultra 5.1 Ate Typ 200 Ford HD AP 550 Brake Man Earl's Hypertemp 421 NEO Motul Racing 600 AP Super 600 Castrol SRF Motul RBF 600 NEO Super DOT 610
Dry BP ºF
(ºC)		 509
(265)		 518
(270)		 536
(280)		 550
(288)		 563
(295)		 577
(303)		 585
(307)		 585
(307)		 585
(307)		 590
(310)		 590
(310)		 593
(311)		 610
(321)
Wet BP ºF
(ºC)		 365
(185)		 375
(191)		 392
(200)		 290
(143)		 293
(145)		 300
(149)		 421
(216)		 421
(216)		 421
(216)		 410
(210)		 518
(270)		 420
(216)		 421
(216)
Price
per liter		 $12-14 ~$14-20 $10-15 ~$6 ~$28 $25 ~$30 $34 ~$18-21 ~$36-40 $65-75 ~$30 ~$30

Ate Super Blue and Type 200 - the best deal in performance brake fluid at $9.95/liter.
Ate Super Blue or Typ 200
Store Price/liter
raceshopper.com   $9.95
Strictly German   $9.95
DaliRacing $10.00
SP Motorsports $10.85
Spec Miata Superstore $10.95
Cobalt Friction Technologies $11.99
Mach V Motorsports $14.00

Acknowledgments

Some of the information presented here was gathered from various books, email lists, message boards, and vendor web sites. I would like to acknowledge and thank the following for their contributions.
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Except for the small gif and jpg images, the content, images, photographs, text, and multimedia displayed are Copyright ©2000-2004 by Jeff Lucius and K2 Software. All rights reserved. No part, section, image, photo, article, or whole of this site may be reposted or redisplayed without permission of the author.
Page last updated August 3, 2004.