[T3] Brake Fluid

Jim Adney jadney at vwtype3.org
Wed Dec 6 15:20:58 PST 2017

Brake fluid is something that most of us very seldom think about, but it's very 
important and there are some things that you should know about it.

Most cars in our era came with DoT-3 brake fluid, but our cars, at least the 
late ones with disk brakes, came with DoT-4, which has a higher boiling 
point. The original stuff was made by ATe (Alfred Teves) and was blue. It 
was beautiful stuff, but it's hard to find and it's pricey once you find it. Both 
DoT-3 and 4 have the same basic chemical composition as antifreeze. It's 
not exactly the same stuff, but it's important to understand that it's not a 
petroleum product. They are completely miscible with water, which makes 
water the ideal fluid to clean them up with.  

These standard fluids work okay, but they have some drawbacks:

They are hydroscopic, so they absorb water out of the air. Because of this, 
they should be flushed and replaced every few years with fresh, dry, brake 
fluid. Mixing brands is no problem.The brake fluid in most cars has never 
been replaced. This causes problems in 2 ways. "Wet" brake fluid boils at 
lower temperatures, the wetter the lower. this means that a lot of hard 
braking can lead the the loss of stopping power when your brakes get hot 
enough to boil the fluid.  

The second problem is that damp brake fluid becomes corrosive and causes 
rusting of the internal surfaces of your brake parts. This is especially a 
problem in disk brake calipers, where a small amount of fluid will seep past 
the seals, get exposed to the air and get "wet" and cause corrosion just 
outside the piston seal.

Those of you who know me know that I've long been a fan of DoT-5 Silicone 
Brake Fluid. DoT-5 is chemically different. It does not mix with water and it 
does not absorb water, so it does not need to be changed, ever. Silicone is 
also a MUCH better rubber to metal lubricant than the other stuff, so master 
cylinder piston seals now last forever. You can buy DoT-5 at your FLAPS.

The downside of DoT-5 is that it is expensive and somewhat more difficult to 
install correctly. Also, because it doesn't mix with water, it's hard to dispose 
of. I recycle it myself and use the recycled stuff to flush the next car, but 
that's more than most people will want to do. I'll take your dirty stuff, but I 
don't know how a good way to ship it would be.

I've used Silicone Brake Fluid in all my air cooled cars since 1977, and I'm 
completely sold on it. It's upside is that it has reduced my need to do brake 
work on my own cars by at least 95%. It has also meant that when I get a car 
out of storage, I never have to worry about whether the brakes will still work.

Note: DoT-5 is NOT a good metal-to-metal lubricant, so it's use in cars with 
ABS is not recommended (because of the ABS pump.)

I have a file on using and installing DoT-5 in your car. Email me if you want a 

So here's the take-home message: Use DoT-5 if you can, but if you don't 
want to do that, use DoT-4 and flush and replace it at least every 4 years.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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