[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 That's been replaced by what appears to be tin 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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