Since DOT 4 fluids were developed, it was
recognized that borate ester based fluids offered the potential for boiling
points beyond the 446F requirement, thus came the Super DOT 4 fluids - some
covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point
of 500F (same as silicone, but different chemistry).
You can go ahead and mix or flush (replace) any DOT 3, DOT 4, or DOT 5.1 brake
fluids, no problem. So there are no special bleeding procedures and you don't
need to run anything in the lines. The only one to watch out for is DOT 5
(silicone brake fluid).
DOT 5 does not damage paint like other brake fluids do, but is not recommended
by AP Lockheed for use in their systems due to higher wear than with other type
fluids.
Brake system contamination:
The single most common brake system failure caused by a contaminant is swelling
of the rubber components (piston seals etc.) due to the introduction of
petroleum based products (motor oil, power steering fluid, mineral oil, etc.) A
small amount is enough to do major damage. Flushing with mineral spirits is
enough to cause a complete system failure in a short time. I suspect this is
what has happened when some owners change to DOT 5 (and then assumed that
silicone caused the problem). Flushing with alcohol also causes problems. Brake
systems should be flushed only with DOT 3 or 4.
If silicone is introduced into an older brake system, the silicone will latch
unto the sludge generated by gradual component deterioration and create a
gelatin like goop which will attract more crud and eventually plug up metering
orifices or cause pistons to stick. If you have already changed to DOT 5, don't
compound your initial mistake and change back. Silicone is very tenacious stuff
and you will never get it all out of your system. Just change the fluid
regularly. For those who race using silicone fluid, I recommend that you crack
the bleed screws before each racing session to insure that there is no water in
the calipers.
British Cycle Supply Company does not recommend use of DOT 5 silicone brake
fluid on British motorcycles equipped with any AP Lockeed disc brake components.
10/01/05