DOT 5 brake fluid

[rockmachine]

I just purchased Prestone Silicone racing brake fluid DOT 5.
My master cyclinder says only use DOT 4.
Will I be safe useing DOT 5??
And can I use it for the clutch Master Cyclinder?

[Mac]

It should be fine, I believe dot 5 surpasses  the spec for dot 4.

It should be fine to use it for the clutch M/C too.


Edited by Mod: Please read on before using DOT 5!

[Headmanbrewing]

I hope I caught you.

DO NOT use DOT 5 in your brake system.

DOT 5 is a silicone based fluid that does not mix with DOT 3 or 4.  DOT 5 is typically used in USPS vehicles because it does not attract water, therefore it does not have to be replaced as often.  It is also used by car collectors and restorers so the brakes do not have to be bled regularly.  DOT 5 is known to have a softer/spongier pedal feel than 3 or 4.

DOT 5 can be used in any brake system, IF you first rebuild or replace every piece of the system that held fluid, like the master cylinder, calipers, lines, etc.

On the other hand, if it is DOT 5.1, that is compatible with DOT 3 and 4.

[rockmachine]

You caught me before I used it.
Thanks for the info. You saved me some serious $$ if I used it.
 

[rockmachine]

Here is what I learned,,

Pretty "dry" info, but here goes.

The primary benefit of silicone based, DOT 5 fluids is the potential boiling temperature. Once the fluid reaches boiling temperature, the steam/vapour created will simply compress and the brakes simply go away.

Compared to glycol and glycol ester based DOT 3 (boiling point 401F), and DOT 4 which has added borate esters that raises its boiling point to 446F, silicone based DOT 5 has a boiling point of 500F.

DOT 3 and DOT 4 are both miscible with water and will, over time, absorb and entrain some amount of moisture, which will reduce the boiling point, hence the need to change it out. Moisture that might find its way into a DOT 5 system will not mix with the silicone, but will concentrate at the bottom because the silicone is lighter than water.

Silicone does appear to keep its viscosity more constant in very cold temperatures, but most of us don't ride in temperatures low enough for that to come into consideration (the US military specs DOT 5, apparently for the cold weather performance).

If silicone is introduced into an older brake system, the silicone can bond with the gunk in the system that results from component deterioration to form a gelatin-like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick.

In a relatively new system the chances of this goop forming are reduced, but it appears that, especially with some newer, higher temp formulations of DOT 3 and DOT 4, there is little advantage to using DOT 5.

If change to DOT 5 in an older system, experts advise that you don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, they recommend that you crack the bleed screws before each racing session to insure that there is no water in the callipers.

There are some new formulations of DOT 3 and DOT 4, known as Super DOT 4(borate ester based instead of glycol) which have boiling points similar to DOT 5 (500F).

So, if you do use the DOT 5, you're basically there for the life of your system, and the newer formulations of DOT 4 offer the same boiling resistance as DOT 5.

[Headmanbrewing]

Good research/info.  Well written.  

[kevin_stevens]

Good research, agreed.  However, it leaves one with the impression that DOT5 is a legitimate option for normal street use.  It really isn't, the downsides are almost insupportable.  First, you have to be sure your manufacturer supports DOT5, many don't.  Second, the fact that silicone fluid is immiscible results in significant corrosion at the low point of the system where the water collects, which in most production bikes is very difficult to evacuate (that's specifically why DOT 3/4/5.1 fluids *do* absorb water, so you can get it out of the system).  Third, silicone fluids are themselves somewhat compressible, and you won't ever get a solid lever.

If you aren't a custom bike builder dealing with multi-thousand dollar paint jobs, you really shouldn't even consider DOT5 usage - certainly not in the context of a "Beginner's Garage".

KeS

[JimWilliamson]

FWIW...

On my '68 Ford wagon, when I replaced the wheel cylinders, I flushed and filled with DOT 5 Silicone fluid. No issues (that I could ever tell). Bought the car with 108K miles and sold it at 257K.

YMMV.

[KokomoSam]

I just had this questions come up recently.  My buddy found this link.  It helps clarify the dot3 vs dot4 vs dot5
http://www.motorcycleproject.com/motorcycle/text/cows-brakefluid.html

I know that this article helped me.

Sam

[1KPerDay]

It should be fine, I believe dot 5 surpasses  the spec for dot 4.

It should be fine to use it for the clutch M/C too.

[Mac]

Hey, as far as I know it is. or was,  or something.