There are just a few choices when it’s time to decide on which brake fluid you want to use, but sticking with the Manufacturer’s recommendation is probably your best choice, especially on newer cars. The choices are pretty much just DOT 3 and DOT 4. Either/or works well. But what about that curious purple tinted DOT 5?
It’s a silicone fluid that has great attributes, such as a higher boiling point, and won’t damage paint. If you spill a little, it’ll simply add a shine! No worries when working on your beloved classic cars! I’ve used it and like it, and no, there’s no ‘softer’ pedal with it. There are drawbacks, however, and the first you’ll notice is the price, as it’s way up there compared to DOT 3 and 4.
DOT 3 and 4 will eventually cause rust problems in your system, as it absorbs water. Water ‘does’ enter your ‘sealed’ system via humidity (as in checking fluid levels) but mostly via your brake caliper piston seals. If the rotors have ‘any’ runout (wobble), the brake pads, which ride on the rotors all the time, transfer this motion to the caliper’s piston seals at a rapid rate and will pull air (with moisture/vapor) into your system. You eventually end up with air in your brake system, even though it does take a long time to get to the stage where the brakes need to be bled of air.
So what, right? Just bleed the system every few years and you’re good to go. Wrong. Adding brake fluid and bleeding your brakes does nothing to remove the accumulated water. All you’re doing is removing air so you’ll have a stronger pedal, yet this will eventually lead to rust and scale and very dirty looking fluid. So then use DOT 5 and no more worries, right? Nope, sorry!
DOT 5 is silicone. It doesn’t absorb water. DOT 3 and 4 ‘does’ absorb water. So which do you want in your system? Well, DOT 3 or 4, of course! Here’s why… Brake fluids all react the same in that none will keep moisture from entering the system. Moisture in a DOT 5 system will gather in pockets as it isn’t absorbed into the brake fluid. It will get trapped, and it will rust. Moisture in a DOT 3 or 4 system will absorb throughout, and that’s actually a good thing. There’s a caveat, of course, and that is that with either system you MUST flush the fluids out at least every 3 years. So NOW, that expensive DOT 5 is not so attractive, is it. However, if you don’t mind the added expense, DOT 5 is still less threatening to any of your painted parts, or you. The choice is yours, but like your momma once said… “don’t forget to flush.”
So Flush this…
It’s easy. Car on jack stands, wheels removed, tubes from the bleeder nipples going into nearly empty containers, remove the master cylinder lid, open all the bleeders and let it gravity drain. When empty, pump the pedal (slowly) a few times to empty the pistons. Add denatured alcohol to the master and go through the same drill. Keep this up until the alcohol runs clear. Blow the system out with low pressure compressed air to help dry things out.
You’re now ready to add fresh DOT ‘whatever’ and bleed the system in the traditional pattern using gravity or one of the various pressurized or reverse bleed systems on the market. There’s always the pedal pumping method as well. Whatever works best for you, just try to do this every 3 years and be confident that your brake system is as clean as it’s going to get. Lastly, if you decide to use DOT 5, make sure it’s never topped off with DOT 3 or 4!! Perhaps a label or tag on the master cylinder lid will do.