Brake Fluid
 

I was going to change my brake fluid today (1998 S70), but I can't find Dot 4+ that is called for in my owner's manual. Is this a dealer-only item? I've been to Autozone, Murray's, even Walmart. No one at the parts stores has heard of it. It's Saturday, so the dealer isn't open.

Thanks again.

The way I understand it, and anyone correct me if I am wrong, is that the "4+" means "DOT 4 or greater". That is because there is DOT 3, 4, 5, and 5.1 available. The last time I was in Autozone I saw two brands of DOT 4 fluid, Prestone and Valvoline. The fluid I got was the ATE super blue from FCP. But in a bind I would probably use either of the other two.

I thought I read somewhere (can't find it now) that the 4+ had something to do with silicone (or some other additive), which differentiated it from the 4. The same thing had to do with the difference between the 5 and the 5.1.

walmart should carry it! Maybe they are out of stock.That's where I got my DOT4 fluid@

I found this on shoptech.com:

Historically, DOT 5-level performance (specifically boiling points and viscosity) could only be achieved with silicone-based fluids. However, modern compounding has created glycol ether-based fluids which now meet DOT 5 bogeys in these key areas. Consequently, the DOT 5.1 moniker was created to differentiate between these two very different chemistries which both meet DOT 5 performance requirements.

In so many words, DOT 5.1 fluids are simply DOT 4-type fluids which meet DOT 5 performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4 fluids without concern. In some circles, they are even referred to as ‘DOT 4 Plus’ or ‘Super DOT 4’ fluids because they are more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 fluid. In fact, DOT 5.1 is essentially comprised of Borate Esters.

While it may not be obvious, the big advantage of the DOT 5.1 fluids is that they contain all of the nifty water-absorbing characteristics of the DOT 3 and DOT 4 fluids while simultaneously providing for very high boiling points and relatively stable viscosity over a wide range of temperatures. The best of all worlds, you could say. The table below sums it up quite nicely.

PROPERTY DOT 4 DOT 5 DOT 5.1
Dry BP (F)@ 0.0% H2O 446 509 509
Wet BP (F)@ 3.7% H2O 311 356 356
Chemical Composition Glycol Ether / Borate Ester Silicone Based Glycol Ether / Borate Ester

(As stated earlier, the table data above contains the minimum properties for a fluid to be called a certain type. For example there are many racing brake fluids with Dry BP performance at or above 590o F and Wet BP at or above 390o F.)

So, what is the downside of the DOT 5.1 fluids? Like most things in life, the good stuff isn’t cheap. DOT 5.1 fluids typically cost three to four times as much to manufacture as a conventional DOT 4 fluids. There’s always a catch…


So why is silicone-based DOT 5 fluid more compressible than other fluids?
On their own, silicone-based DOT 5 fluids are entirely different animals than DOT 3 and DOT 4 fluids. In addition to having characteristically higher dry and wet boiling points, they also tend to have much, much lower viscosities. In other words, they flow more easily relative to temperature.

One side effect of this chemistry is that there is more “room” for air to fit in-between the individual molecules of brake fluid than in DOT 3 or DOT 4 fluids. Note that we are not talking about big bubbles of air here which are visible to the naked eye, but rather microscopic amounts of air which are finely dispersed (entrained) in the brake fluid matrix.

Now, all fluids have a certain amount of compressibility to start with, but adding even the smallest amount of air into the solution can dramatically increase the amount of elasticity in the system. In the case of silicone-based fluids, air is quite happy to take up residence between the brake fluid molecules, and as a result the fluid compressibility goes down. This is felt at your foot like stepping on a big spring. As you can imagine, more air = more spring.

Dot 4 is everywhere.

Autozone, Murrays, Walmart, Advance Auto all sell it. Then go to the dealer and get if as you only need a single bottle of it. That will almost bleed two cars out.

Made a huge difference in my pedal feel and braking power. I bled mine last weekend.
Did it with a vacuum pump and it really helped.

My question was regarding the DOT 4+ spec called for in the owner's manual. It sounds like everyone uses 4 in their cars without problems.