All,
Further to the excellent article on brake fluids at the VTR
site (maintained by Ken Streeter...thanks Ken), it does not mention
the relatively new 5.1 brake fluid. The following article came out of
January, 1995 Motorcyclist magazine.
BRAKE FLUID NUMBERS GAME
========================
A parts person at my dealer recommended switching to DOT 5.1 brake fluid,
but when I asked him about it, his information was confined to a weak *It*s
supposed to work better.* Can you tell me about his stuff? Is it
compatible with silicone fluids? Is it compatible with DOT 3 or 4? Will
it etch paint? Does it absorb water? It is better, why?
Cal Lampton
Hermosa Beach, CA
The DOT could avoid a lot of confusion by giving this new fluid a
different designation. The 5.1 designation could lead one to believe that
it's a modification of silicone-based DOT 5 brake fluid. Calling it 4.1
or 6 might have been more appropriate since it's a glycol-based fluid like
the DOT 3 and 4 types, not silicone-based like DOT 5 fluid. (In fact,
Spectro is marketing a similar new fluid which they are calling Supreme
DOT 4, which seems less confusing.)
Since 5.1 fluids (and Spectro's Supreme DOT 4) are non-silicone
fluids, they will absorb water (though apparently not quite as quickly) and
attach paint, but they won't create the slight mushiness in your brakes
caused by the air that never seems to be entirely purged from silicone-based
DOT 5 fluids.
DOT 5.1's claim to fame is superior performance; it has a higher
boiling point, either dry or wet, than DOT 3 or 4. In fact, its dry
boiling point (about 275 degrees C) is almost as high as racing fluid (about
300 degrees C) and 5.1's wet boiling point (about 175 to 200 degrees C) is
naturally much higher than racing's (about 145 C). It also doesn't absorb
moisture as quickly as racing fluid, which absorbs water very quickly.
Compared with DOT 3 or 4, there is no apparent disadvantage, and any
brake system components suitable for 3 or 4 should be compatible, with the
new fluid.
If you decide to use 5.1 or Supreme 4, you can flush your brake system
normally, if you are currently using DOT 3 or 4. Soak most of the old
fluid out of the reservoir with a rag or syringe, then add the 5.1 to the
reservoir and let it push the old fluid out by bleeding the system.
It's likely that 5.1 won't be compatible with the seals in systems
built for silicone-based DOT 5, such as those in Harleys. The seal
materials were selected for silicone. Ask the manufacture before making the
change. If you are satisfied that the system won't be damaged by 5.1 and
you decide to change, you have two options. You can take it completely
apart and clean everything with denatured alcohol, then fill it with 5.1
after reassemble. If you want to avoid disassembling the system, buy a
couple of cans of the cheapest DOT 3 fluid you can find (preferably a
different color then either of the other fluids). Mop out the master
cylinder, then use the cheap fluid to flush the system. It will probably
take 500 ml or so to do this. When the silicone fluid is gone, add the 5.1
and bleed until it's the only stuff in the system.
As with 3 and 4, the 5.1 fluid should be changed with three years.
Shane Ingate in San Diego
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