[TR] [EXTERNAL] Re: Brake fluids again

Randall TR3driver at ca.rr.com
Thu Apr 16 11:29:14 MDT 2020


This is a debate that has been going on for literally decades.  To be quite
honest, my own opinion has changed somewhat over those decades.

You certainly can convert by just draining, refilling with DOT 5, and
bleeding each corner until you get clean purple fluid.  Although the two
fluids will not mix (kind of like oil and water), they are fully compatible
and can co-exist peacefully in the same system.  And the result, IMO, will
certainly be better than not converting at all.  I've done it myself several
times and always been happy with the result, at least at first.

But, there are two drawbacks that I see to this approach.  One is that there
are little pockets of DOT 3 or 4 that remain in the system and continue to
do what DOT 3/4 does, ie absorb water, deteriorate, and become corrosive.
It also retains it's lower boiling point (which continues to go down with
time).  If you read through the Holbrook paper, they had a test failure (a
73 MB 200) that lost it's brakes during the test, and they attributed that
failure to having only 4% of conventional brake fluid still in the system.
They retested the same car after "properly" converting to DOT 5 (which
presumably means removing that last 4%) and got better results.  

The other is that sometimes, not often and certainly not always, I have seen
sudden failures with "rubber" components that were used for a long time with
DOT 3/4 and are now exposed to DOT 5.  You can certainly argue that they
were old and about to fail anyway; but old seals generally just get hard and
don't seal as well (slow leaks).  They don't turn soft and tear, suddenly
rendering them useless.  

Just for an example of what I'm talking about, I converted Stag #1 to DOT 5
shortly after buying it, by just bleeding through.  Worked great at first,
but months later the brake pedal started sometimes slowly sinking to the
floor at a stop light.  At that time, I didn't want to rebuild the MC
myself, so I made an appointment with a shop in Pomona that was supposedly
familiar with Stags.  On the way to Pomona, within just a few miles of each
other, both cups in the MC failed in the same exact fashion.  They turned as
soft as Gummi Bears, and tore through the base of the seal.  This is not
normal seal wear!

So, my current opinion is that, if the choice is just to "bleed through" or
not convert, it's certainly better to bleed through.  But what John Deere
(my former employer) calls "best practice" is to clean the hard lines and
replace all the seals and hoses with new.  The result is more likely to give
you decades of service with no further problems.

As a side note, DOT 5 also gives a very slightly softer pedal than DOT 3/4
does.  You can compensate for this effect by converting the soft lines to
the "braided SS" lines, which use a much harder tube inside and hence don't
swell as much under pressure.  I got the ones for my TR3 at Tsi in Ohio; the
Stag lines came from Rimmers.  ISTR Ted later told me that they can make
them up for Stags as well.

-- Randall

>  Can I 
> drain out the existing DOT 4 and switch to DOT 5? 
> 
> Or do I need to completely flush the entire system and 
> replace all the wheel cylinder, master cylinder, and clutch 
> master and slave cylinder seals when converting from DOT 4 to DOT 5?  
> 



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