J.C. Hassall wrote:
> I've replaced all components in my brake system, with the exception of
> the brake light switch, which is actuated by hydraulic pressure. Since
> it was in contact with DOT 3 a decade+ ago, I think it's a potential
> fluif leak after I fill the system with DOT 5. What's the list.wisdom?
>
> TIA
>
> Jim
As I read Nelson Riedel's article at Buckeye Triumphs, using Silicone
and Glycol based fluids at the same time does not hurt anything. They
don't mix with each other but don't cause any harm.
From Nelson....
Compatibility of Silicone Fluid with Seals and Hoses: The rubber seals
in the brake system swell in the presence of brake fluid thus
improving the sealing properties. The silicone fluids Browning
reported on in 1974 met established swelling requirements for SBR
cups, ethylene propylene (EP) seals, natural rubber cups, and Neoprene
brake hoses. A tentative specification had been published for such
compatibility prior to that report. So, no problem with compatibility
with the seals. Apparently some earlier seal failure attributable to
silicone fluid (pre 1974?) were traced to early swelling agents. I
had a recent discussion with one of the mechanics at a local military
maintenance depot. He said they had some seal problems with an early
yellow silicone fluid. He said the current fluid is purple and they
are having no trouble. I don't know the era of the yellow silicone
fluid but I'm pretty sure the silicone fluid of the late 80s was
purple. Of course, the DOD might have had a special formulation.
Compatibility of Silicone Fluid with glycol-based Fluids: The two
types of fluid in fact don't mix (they are not miscible). If mixed
and then allowed to stand, they separate into two layers, like cream
on milk (for those old enough to remember non homogenized
milk). Browning reported on extensive tests of running systems with
50/50 silicone/glycol mixtures. Several other reports of tests of
mixed fluid systems are available on the Internet. Every case I've
found reported no failures. Being an engineer, I figured I should run
a little test of mixing the fluids and see what happened. The
following shows the result from my basement laboratory.
Summary: So what have I learned:
* Silicone based DOT5 fluid has a higher dry boiling temperature
that DOT3 & DOT 4 fluids.
* All glycol-based fluids (DOT3 DOT4 & DOT5.1) can quickly absorb
moisture that lowers the boiling point to the wet minimum DOT
specification or in many cases even lower.
* In one study virtually all glycol-based systems examined after
24 or more months use had the 3.5% or greater moisture content used
for the wet boiling point specifications. In some cases, this much
absorption occurred in as little as a few months as the vehicles sat
on new car lots.
* Silicone based DOT5 fluid has a much higher real wet boiling
point than any of the glycol-based fluids discussed here because it
absorbs very little water and the wet boiling point is essentially the
same as the dry boiling point.
* Silicone based DOT 5 fluid prevents entry of moisture and salt
into the brake system through the hoses.
* Silicone based DOT 5 fluid was reported to be compatible with
all types of brake seals way back in the the early 70's.
* Silicone based fluid doesn't mix with glycol-based fluid.
* Extensive testing of brake systems with 50/50 mixes of silicone
and glycol-based fluids have shown that the mixtures performed
properly with no failures recorded.
* Silicone based DOT5 fluid is slightly compressible but the
compressibility is manageable at temperatures below which the standard
Glycol fluids vapor lock (wet boiling point).
* Silicone based fluid retains small suspended air bubbles. This
requires extra care when filling the reservoir and may require a
subsequent bleeding a few days after first filling the system to
extract any air bubbles that remained in suspension during the initial
bleeding.
* Glycol-based fluids attack paint whereas the silicone fluids do not.
My Conclusions: The conclusions I draw from these data are:
* The glycol-based fluids give satisfactory performance in the
light duty use I give our relatively new autos. Because the fluid
absorbs moisture and becomes contaminated quickly I plan to make it a
practice in the future to change the brake fluid at the same time I
flush the cooling system, every three or four years.
* If I lived in a mountainous area where the brakes might be
subjected to high temperatures I'd definitely change glycol fluid at
least once a year or switch to DOT5 silicone fluid, keep the rear
brakes in adjustment and renew the pads when worn to half new
thickness.
* If I were racing I'd use a glycol based racing fluid and change
out the fluid before every race.
* I will use DOT5 silicone fluid in all my TRs in the future to
prevent paint damage and to protect the hydraulic systems from
contamination and corrosion.
Switching fluid type: Given that I've decided to switch fluids in my
TRs, how and when do I plan on doing it? I guess the first point to
make is that switching fluid will not fix leaking seals or uncorrode
(a word I just invented) corroded pistons or cylinders. So, if the
brakes have a problem, fix the problem and that is then an excellent
time to switch fluids. (I just thought of another point -- some folks
have their cylinders bored and then sleeved with brass to minimize
corrosion. The use of DOT5 fluid will likely eliminate any future
corrosion and is much less effort & expense than brass sleeves.) I
switched fluid in my '76TR6 last summer when I had the master cylinder
off as part of the repainting project. I will do the same for my
TR250 when I repaint it this summer. I guess if I had a TR with good
paint and glycol fluid and knowing what I do now, I'd switch the fluid
ASAP ---- in a few weeks or less. The cost and frustration involved
to repair damaged paint is just too great.
What I actually did: So --- what did I do with my '76TR6? As
mentioned previously, I had the master cylinder and PDWA off to powder
coat them to improve the appearance. I used compressed air to blow
out the lines and wheel cylinders. In the case of the calipers that
can store considerable fluid, I pressed the pistons all the way back
into the calipers to force as much of the old fluid out as possible.
I then reinstalled the master cylinder, PDWA and interconnecting
lines, filled the system with silicone fluid and bled the system. I
bled the system again in a week or so and detected no air exiting the
bleed nipples nor observed any subsequent system improvement. The
DOT5 has been in the system for a year and I'm very happy with the
performance. I can see no difference between the operation of that
system and my TR250 equipped with DOT4.
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