Locate your brake fluid resevoir. It should look similar to the
picture. The recommended level is usually at the seam but should
be indicated on the side. Note if the level is at the seam it is
hard to see so you might try lightly rocking the resevoir to move
the fluid inside.
that the fluid level is at the required level and top up with the correct
fluid as necessary. Note if your fluid is low then you might have a leak
somewhere and it must be checked. Check you levels at regular intervals
to catch any such incident immediately.
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:
Boiling point, deg F (minimum)
boiling point, deg F (minimum)
*DOT 2 specifications
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: Musn't freeze or thicken at cold temperatures.
Must not compress.
Must flow freely thru rather small passages.
Must not corrode or react with materials in the brake system.
Provide lubrication to the moving parts of the brake system.
Its properties must remain stable for extended periods.
Compatible with other glycol fluid chemistries.
Cannot decompose or form gum or sludge in the system.
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. 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
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
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 quid. 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.
Thanks to the "Brake Handbook" by Fred Puhn, HPBooks, ISBN:
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.