DOT 3 and DOT 4 are functionally very similar. 4 has a higher boiling
point, but motorcycle brake systems don't generate the kind of pressure and
temperatures that need it on the street, in general, although it certainly
won't hurt your system to put it in.
DOT 5 is very different--it's silicone based, doesn't absorb water, isn't
corrosive, is bad for some seals, is hard to bleed, and is not miscible
with 3 or 4. Stay away from it--it needs a system designed for it. And it
comes as stock item in Harley's, so it must be terrible. :->
DOT 5.1 is compatible with DOT 3 & DOT 4 (If I ever get hold of the
bonehead who named DOT 5.1...)
Here's more detail than you really want:
DOT3 is an aliphatic polyether.
DOT4 is borate ester based.
DOT5 is polydimethylsiloxane (silicone based).
DOT5.1 is borate ester based, thus its compatibility with DOT3 and
More information can be obtained from the following standards documents:
DOT3: SAE J1703
DOT4: FMVSS 116; proposed SAE standard J1704
DOT5: SAE J1705
DOT5.1: No SAE spec
If you are interested in obtaining copies of these standards documents, you
may order them directly from SAE at
According to DOT Federal Motor Vehicle Safety Standards specification
49CFR571.116 (which refers to SAE documents J1703, J1704, J1705), the
minimum equilibrium reflux boiling point requirement in deg C for each is:
DOT 3 205
DOT 4 230
DOT 5 260
DOT 5.1 260
This shows that, all else remaining the same, DOT 5.1 has a significant
advantage in heat capacity over DOT 4. Note that these specifications are
for completely dry (no H2O content) brake fluid.
Of course, all else does not remain the same and other than boiling points
and H2O content (which is very detailed in itself), most other properties
were beyond the scope of testing/interest of my friend. Any other
information should be gained from SAE, DOT or other authority.
In other words....we should use DOT 4 or DOT 5.1 NOT DOT 5!!!