Modern road and race car brake fluids are primarily Polyalkylene Glycol Ether, or just glycol for short. All modern glycols should carry a DOT 3 or DOT 4 rating. DOT 5 is for silicone-based fluids. To meet DOT approval, the fluid must meet the following boiling point specifications:
| DOT 2* | DOT 3 | DOT 4 | DOT 5 | |
|---|---|---|---|---|
| Dry Boiling point, deg F (minimum) | 374 | 401 | 446 | 500 |
| Wet boiling point, deg F (minimum) | 284 | 311 | 356 |
* DOT 2 specifications are obsolete.
NOTE: If you have brake fluid sitting around that's DOT 2, SAE J1703, SAE 70R3, or SAE 70R1 (for drum brakes only), toss it. These old fluids may not be compatible with the sealing materials in modern brake systems, and besides, they're probably full of water from sitting on the shelf all these years.
The reason we like brake fluid not to boil is simple: Brake fluid is subject to high temperatures in the brake calipers, so when it boils, its chemistry changes permanently and it liberates small bubbles of gas. These bubbles collect and become trapped in the system. Since gasses are compressible, we now have a soft, or spongy brake pedal.
Besides not boiling, brake fluid needs to have some other specific qualities:
That's quite a lot to ask of a fluid, no? Most of the fluids I can easily bring to mind; water, oil, anti-freeze, good single-malt Scotch, each possess only a few of the qualities listed above.
Glycol based fluids are hygroscopic, meaning they absorb water. Over time, water is absorbed and the fluid's boiling point drops. A typical glycol DOT 4 fluid that starts life at 450F dry boiling fresh from the can will degrade to 300F at 12 months by just picking up 3% of its weight in water. By 24 months its likely to be well below 300F at 5% water. In modern brake systems, most of the water absorption occurs in the plastic reservoir on the master cylinder. Poly-based plastics are slightly water permeable, so even brake fluid packaged in plastic bottles is subject to water absorption. Metal cans are superior, Ford Heavy Duty DOT 3 ("Preferred by Racers Everywhere...") is packaged in metal, as is Castrol SRF ($70 per liter!). If you're buying fluid in plastic, try to find a date code on the bottle, get the most recent batch if you can decode the numerical mess.
Racing brake fluids with very high dry boiling points, say, above 500F out of the can, tend to degrade more quickly then more mundane fluids. A 400F dry BP DOT 3 fluid will drop to about 325F wet BP with 2% water by weight. A 500F dry BP fluid will also be at 325F wet BP with just 2% water. Racers bleed/flush their brake fluid pretty frequently, at least every race, sometimes between heats, so they're not too concerned with wet boiling point. If you decide to run a high zoot racing fluid in your street car, be prepared to flush it more often to maintain a firm pedal. ( Contrary to popular opinion, the brake pedal cannot be made firm by smearing it with Prolong. )
Silicone brake fluids are not hygroscopic, and tend to retain their dry boiling points for very long periods of time. For this reason, silicones are favored by owners and restorers of classic and antique cars, as there is minimal danger that seldom-used and possibly irreplaceable brake components will be lost to corrosion.
Silicone will cause natural rubber to swell, even when it's compounded with synthetics. The seals in modern brake systems are no longer 100% natural rubber, but blends of natural rubber and synthetics like nitrile. Glycol fluids will also tend to swell blended rubber seals, but to a much smaller degree then silicone. Swollen seals may leak, or cause caliper pistons to bind, resulting in brake drag.
Silicone has several other properties that make it less then desirable for street or track use. When forced thru small orifices under high pressure, like the solenoid valves in an antilock brake system, it tends to foam, generating bubbles. Bubbles in brake fluid make for spongy brakes. Silicone also tends to become slightly compressible at temperatures near its boiling point, which makes it generally inappropriate for racing.
To get the maximum benefit from silicone, the entire brake system MUST be flushed of old glycol fluid. A brake system cannot be completely flushed using the bleeder fittings, as they are purposely at spots in the system to allow air to be bled, you simply can't get all the old fluid out by bleeding. The best way to completely flush a brake system is to dismantle and overhaul it, cleaning everything with alcohol, and then coating all the parts with the new fluid as they are re-assembled. Going to this much hassle just doesn't justify changing to silicone, IMHO.
WARNING: do not clean brake system components with petroleum solvents as they will contaminate brake fluid. Do not lubricate brake system components with petroleum greases or oil, use fresh, clean brake fluid as an assembly lubricant.
As for the $$$$ issue, why not just flush the brake fluid yearly? A quart of Castrol GT-LMA DOT 4 will set you back about 6 bucks. Buy a one-man brake bleeder kit while you're at the auto parts store. The actual flushing operation should take about an hour, even for the un-initiated. This is relatively cheap insurance to guarantee the health of such a critical component on your automobile. You'd be surprised by the number of 10 year old cars on the road running the same brake fluid that the factory put in.
BTW, I recently ran across a distributor for ATe Super Blue Racing brake fluid, hadn't seen this stuff for quite some time. (ATe supplies OEM brake components to Ford.) Its DOT 4, 536F dry boiling, packed in metal cans, and comes in 2 colors, blue and amber. This way, each time you bleed, you use a different color, so when the fluid changes color at a particular caliper, you're done. Imparts has it at $13/liter, 2 liters for $24. Imparts is at 800-325-9043.
Thanks to the "Brake Handbook" by Fred Puhn, HPBooks, ISBN: 0-89586-232-8
I am now going home and will throughly investigate the long term stability and lubricating properties of a good, single-malt Scotch. I suspect, that in sufficient quantities, its 'stopping power' is formidable.
Last modified July 23, 1997.
Mike Schwartz / mikesc@xnet.com