Brake Fluid

[John N. Packard]

Is there a simple way to tell if a brake system has DOT-3 or DOT-5 fluid?

jnp

[nearchoclatetown]

That's a good question John. I had the displeasure of finding out what happens when the 2 are mixed, no brakes at all!! There was a jell in the master cylinder in a shop truck. Someone finally admitted mixing fluids.

[Guest JT]

When it's new the dot 5 is purple.

If you're not sure you could take a sample and mix 50/50 with water in a container and shake. After a few minutes if it's silicone it will separate out where as the dot 3/4 will stay in solution.

[John N. Packard]

Thanks JT,

I wondered if that might not be a good test to mix with water.

jnp

[Guest De Soto Frank]

You could also dribble a few drops on a painted (unimportant) surface, and see if the fluid softens the paint; supposedly, DOT-5 will not harm paint. DOT-3 will soften most paints.

[Amphicar BUYER]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">You could also dribble a few drops on a painted (unimportant) surface, and see if the fluid softens the paint; supposedly, DOT-5 will not harm paint. DOT-3 will soften most paints. </div></div>

I am not sure I'd do the "drop" test. Do the visual (look for purple) or mix test. DOT 3 will harm paint, DOT 5 does not unless your painting. The silly-cone in it make for lots of problems with new paint like Fisheye.

DOT3 and DOT4 type brake fluids are made from Polyalkalene Glycol Ether, which has a strong attraction to water. They also like to eat paintwork. Water contaminating brake fluid however, causes the boiling point (BP) of the fluid to reduce. DOT3 has a BP of 441F and DOT4 of 446F.

DOT 5 on the other hand has a boiling Point of 500F. The other advantages that DOT 5 brake fluid has is that it is silicone based and as such does not mix well with water. This maintains a high BP and allows better performance. It also does not eat your paint if spilled.

However before going out and changing you brake fluid, you need to be aware of all of the issues associated brake fluid. Because water mixed with DOT 3 & 4 it just weakens the mixture, whereas with DOT 5 it will allow a reservoir of water to sit against your callipers and/or master cylinder which will allow it to rust quicker. Also water has a BP of 212F, which means that if a pocket sits against your callipers, a vapour lock could form much quicker and cause failure to occur at a much faster rate. Additionally DOT 5 costs more and is NOT compatible with DOT 3 & 4. Experts say DON?T mix DOT types at all when you are topping up your brake reservoir or changing your fluid.