[T3] Silicone brake fluid V. brake switches...
Jim Adney
jadney at vwtype3.org
Thu Sep 8 14:09:30 PDT 2011
On 8 Sep 2011 at 11:11, Frank Meek wrote:
> I am not taking sides in this debate, (although, I will confess that I use
> the DOT 4 formulation), but I remember several folks whose OEM, (and after
> market), switches were repeatedly failing. I will further say that my '56
> Porsche Carrera will also be using DOT 4. This Porsche site link is offered
> as *information only* for these aging cars.
> http://porsche356registry.org/356talk/1/27895.html
> This should fire up some opinions.
;-)
An interesting thread that perfectly illustrates how much
misinformation is running around out there.
I have also taken apart lots of these failed switches, and the first
thing I noted is the complexity of the 30 year old German switches
compared to the simplicity and cheapness of most/all the modern ones.
I have Meyle and Euromax here now and have not had a failure with
either one yet, but I don't really have much time on them either,
only about a year.
I cut apart one of the earlier Meyle switches, the ones that were
made with the wrong pinouts, and they were the same poor construction
that I see everywhere else. I've not taken apart one of the revised
ones, but I don't expect it to be made any better inside.
One major difference is the presence of silver contacts in the OE
German switches. That's been replaced by what appears to be tin
plating on the modern ones. Tin is not a good contact material and we
should not expect long life from a tin plated switch contact.
Yes, I've seen melted plastic parts in modern ones, where the OE
German switches had a complicated 3-part metal assembly that never
failed.
The comment about Silicone making a poor lubricant is exactly
contrary to my experience. I had been using Castrol LMA DoT-4 and
rebuilding my MCs every 2 years. That was ridiculous. I've since read
that the key is to do a final hone with a finer stone, but switching
to DoT-5 also did the trick. My '73 started to leak out the mouth a
couple years ago. I figured it was worn out after 20 years of DoT-5
and was prepared to rebuild it. Instead, I found that some rust had
formed at the mouth of the MC, which lifted the seal and let fluid
out. The seal itself was still perfect after 20 years of daily use.
I polished out the rust and put the old parts back in. It's still
working fine.
It's certainly possible that silicone brake fluid can seep past the
seal in the switches and get on the contacts in a way that standard
fluid does not. It's also possible that the little spark that occurs
when you open the contacts makes a byproduct of the silicone that's
an insulator. But frankly, I don't think we need a better explanation
than the cheap tin plating on the modern contacts.
What we really lack here is good new OE style switches to test this
on. The fact that an OE switch failed a few years after switching to
DoT-5 doesn't really tell us anything. The problem is that we're
working with a very small statistical sample, and that sample is made
up of switches which were already 20-30 years old when the test
began.
Compressibility has always been a big question. It's possible that
silicone is more compressible than DoT-4, but the folks at Dow-
Corning say otherwise. The people I talked to were engineers, not
salesmen, so I don't think they would lie about it, and this fact
just came up as part of a larger discussion.
One thing that often confuses people is that they get used to their
system as it deteriorates over time. The pistons get corroded in
their bores and don't retract as far. When the system is rebuild or
replaced it's common to hear a complaint that the pedal is now "soft"
or low. I believe it's now back to normal. People sometimes consider
this a silicone problem when it's actually a new system "feature."
BTW, I put DoT-5 in my '71, with it's original switches, in about
1978. Those switches were still good when I parted out the car about
10 years later.
Finally, there's no reason to use DoT-5 in race cars. Cars like that
get taken apart rather regularly and the fluid all gets replaced
anyway. Under those conditions DoT-4 is cheaper, works perfectly
well, and is actually more environmentally friendly since you can
just flush it down the drain.
--
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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