[Roadsters] Datsun-roadsters Digest, Vol 2, Issue 75

[Steve Jacobson]

DOT4 is upgraded DOT3, better dry & wet(?) boiling specs. Should be
no difference in pedal feel at normal brake temperatures.  It too is
hygroscopic (attracts water).  There are various brake fluid tests
available, with purported boiling points and "hygroscopic-ness".

DOT5 is silicone based, not hygroscopic so does not promote rust,
prone to trapping tiny bubbles (like be careful when pouring in),
hence can be spongy. Not compatible with DOT 3,4,5.1, and if system
is not thouroghly cleaned of those, can cause weird dangerous
behavior.

DOT5.1 is synthetic, DOT 3&4 compatible.  Higher yet dry & wet
boiling points.

I use Motul 600 (DOT 5.1) in my race cars, holds up well, best feel
to me, about $10/bottle (?.5 liter?, 12 or 16 oz). I have liked it
the best. Don't have to worry about wet as it is changed very often.

Lots of people (racers, especially circle trackers) swear by FORD
competition DOT4.  Really cheap, real good.  Must be gotten at Ford
dealer.

For the street, a good DOT 4 should suffice. I have used the
Valvoline synthetic, seems OK.

As long as you don't let it sit forever, s/b OK. A change every 2
years should be quite enough to keep things clean (IMHO).

Speedbleeders make the job easy, as well as a vacuum bleeder.  I have
not had real good luck getting stubborn bubbles out with a
pusher-type  bleeder.  I think you need one of the good professional,
tall ones, not the little bottle types maybe.  I also built a power
bleeder, works, but as I say, doesn't get out the stubborn bubbles. 
I'll not even attempt to go into the supposed physics of why that may
be, cuz I can't prove it.  They just haven't seemed to work as well
for me.

--------------------------------
Its been a while since I have worked on my roadsters, but I have some
questions: Forever DOT3 was the standard brake fluid which is
 hygroscopic
(absorbs water from high humidity air) which then ends up rusting
 your brake
calipers. Some one told me about DOT 4 which was silicone based & was
 not
hygroscopic so rusting wasn't a problem for a seldomly driven car.
 When I
changed over to "4" my brakes were extremely spongy & was accordingly
 was a
problem with this type of brake fluid. I even built a pressurized
 auto
bleeder so I could bleed my brakes alone but resulted in no reduction
 in
sponginess. So what is DOT 5 & 5.1? Do these new fluids address the
sponginess or rusting problem?

Steve
 
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