DOT 3, DOT 4 and Dot 5.1 are:
• Fluids made from Polyalkylene Glycol Ether.
• Hydroscopic, they will absorb water.
• Bad for paint.
• Almost colorless when fresh and turn almost black after absorbing water.
Silicone fluids are:
• Made from silicone oil, Diorgano Polysiloxane.
• Hydrophobic, will not absorb water.
• Sold commercially under the name “DOT 5”, DOT 5 is a brand name not a DOT standard.
• Available in multiple colors, clear, brown or purple (purple will fade to clear over time).
How do I know what I have?
Water is the trick! Pull a sample from your reservoir (poke a soda straw into the reservoir fill hole, put your finger over the top end and pull it out), drop it into a suitable container and mix a little water in it. Water will mix with the DOT 3 and float in DOT 5 (like italian salad dressing before you shake it). Just that simple. In the photo below I mixed fresh DOT 3 with water, I mixed fresh DOT 5 (happened to be brown colored) with water, and I mixed a sample from my reservoir with water. The sample did not mix, so is silicone. By the way, the fluid in my reservoir was originally purple DOT 5 and faded to clear after a year or so.
What happens if I mix DOT 3 and DOT 5....
the world as we know it will end!!! Well, maybe not. They will not mix (see the pic below). I’ve heard that over time they will turn into sludge, I don’t know. I think that you should use one or the other, but not both.
What is the big difference between DOT 5 and DOT 3 in a military vehicle?
For our purposes DOT 3 being hydroscopic or DOT 5 being hydrophobic is the big difference and mostly related to corrosion of brake system components caused by moisture in the fluid.
How is all of this water going to get into my brake system?
Fording can force water into the system (past seals, etc), condensation will allow water in (vent tubes, etc) and in the case of DOT 3 moisture will be directly absorbed from the air.
What’s going to happen because of all this water?
In the case of DOT 3, all the metal parts will corrode and eventually fail (tear up wheel cylinder seals because of pitting or rust through). With DOT 5, if you get enough water in it, the water will “pool” in a low spot and will cause corrosion in that location.
Which one is better?
There are a lot of opinions about which is better. DOT 5 is pretty much maintenance free, DOT 3 needs to be flushed and renewed every so often (if it’s real dark colored it’s time). For me, the military switched from DOT 3 to DOT 5 for a reason, and I’m not going to second guess them. I use DOT 5. I also use DOT 5 in most of my cars. Even though I don’t drive some of them much, since DOT 3 will absorb water from the air I would still have to “flush” it occasionally, not so with DOT 5. The exception is cars with ABS brakes, DOT 5 will “foam” (get little air bubbles in it) and is not compatible with ABS because of this.
So, I want to change from DOT 3/DOT 5 to the other and/or I’ve got a mixture of both.
The best way is to take everything apart and use compressed air to blow everything out, You can flush a DOT 3 system with alcohol (methanol from the hardware store) before you blow it out to get it real clean. That’s a lot of work! The shortcut I take, since I only use DOT 5 (never switch to DOT 3), is to hook up a power bleeder filled with DOT 5 and bleed each wheel cylinder until DOT 5 comes out (use a DOT 5 that is a different color than your DOT 3 so you can see it). Then drive the truck awhile and bleed again (any residual DOT 3 seems to be heavier than DOT 5 and ends up in the wheel cylinders). You might need to do this step more than once if you get a fair amount of DOT 3 when you re-bleed.
Here are instructions for building a real nice power bleeder