The main troubles with mixing DOT 4 and DOT 5 involves water. The two types
do not mix, so if there is any DOT 4 fluid in the system, it will tend to
collect into "bubbles" of DOT 4 fluid. That in itself is no big deal. The
trouble is that the DOT 5 doesn't absorb water, the 4 does. So these little
bubbles of 4 act as sponges, soaking up the atmospheric water that collects
in the system. The water greatly reduces the boiling point, so if the water
logged bubble is near a calpier or wheel cylinder, it can boil into steam
under braking, giving a soft pedal. Also, as the concentration of water in
the DOT 4 bubble goes up, so does the chance of corrosion around that area.
Offhand, I wouldn't worry about it, I doubt there's enough DOT 4 to cause
any trouble.
mjb.
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