Correct brake fluid

[OUbesto]

Hey group, I have a '70, c-10. PO said he replaced front drum brakes with discs from a '71. Original drum brakes are still on the rear wheels. I want to replace the brake fluid since its been in the system for past 9 years. which brake fluid do I use? It seems DOT3 might be the way to go, but.....I just want to get the "experts" and your opinions.

Thanks.

[71CHEVYSHORTBED402]

DOT3, assuming he didn't replace the entire brake system and used DOT5.

[OUbesto]

Thanks Tony. Unfortunately PO passed away so I cannot ask him. Is there a way for me to "test" to determine if he used DOT3 or 5?

[MySons68C20]

https://blog.amsoil.com/dot-3-and-do...he-difference/

[Rust_never_sleeps]

Originally Posted by OUbesto

Thanks Tony. Unfortunately PO passed away so I cannot ask him. Is there a way for me to "test" to determine if he used DOT3 or 5?

As long as you flush out the old stuff thoroughly, it won't matter.

BUT, if you really wanted to know, you COULD suck some out into a jar and add water.
If it's 3 or 4, it'll mix, while 5 will stay separate. https://www.youtube.com/watch?v=7gSchzhcslc

[OUbesto]

Good stuff! Thanks

[71CHEVYSHORTBED402]

Originally Posted by Rust_never_sleeps

As long as you flush out the old stuff thoroughly, it won't matter.

BUT, if you really wanted to know, you COULD suck some out into a jar and add water.
If it's 3 or 4, it'll mix, while 5 will stay separate. https://www.youtube.com/watch?v=7gSchzhcslc

Good chance you know better than I do, but from what I've heard there's no mixing DOT3 and equivalents with DOT5, because that can lead to brake failure, simply because they don't mix, they separate, bubbles and such.

Seems to me you can't flush a system because some of the former will be left in the system, or in other terms, everything that has brake fluid running through it has to be replaced.

For what worth, I saw a video with an engineer demonstrating why DOT5 should not be used in old parts, even if they're rebuilt and clean. That said, others swear by DOT5, so who knows.

[dmjlambert]

I don't believe it would cause a serious problem if a couple of teaspoons of DOT5 somehow managed to get left in the system after pushing a quart or so of DOT3 through the bleeders. But I also think it is likely there would be no DOT5 remaining. When I change out the fluid in my brakes because I want all fresh and all the same brand and type, I feel pretty confident all of it has been changed. My method is to empty the master cylinder with a spoon and rags and pour the new fluid in there, and open the bleeders at the wheels one at a time, and slowly and steadily press the brake pedal numerous times. I periodically top off the master cylinder with new fluid. I attach a clear hose to the bleeder and extend the hose into a small amount of brake fluid in a can or jar. After several brake pedal presses, all the air will be out of the hose and no new air can get into the hose. After I have about 1 cup of fluid in the jar, the new fluid coming out of the bleeder looks nice and clean and the same color as what I pour into the master cylinder. All the fluid has been changed. If you want extra confidence or extra clean flush out, repeat the process on all 4 wheels and push another quart through there and into the jar. It's a one-person job, no helper is needed.

[Palf70Step]

DOT 5.1 is compatible to DOT 3 and will not cause issues if mixed. DOT 5.0 is the odd ball one as far as brake fluid is concerned. It is good, but you do need to have the old system cleaned/Flushed out to not have the chance of issues.

[dmjlambert]

DOT5 is purple fluid. On my truck, when I flushed it for the first time in a long time, the fluid that was coming out of the bleeders was purple, but I was the one who originally filled it and I remember using DOT3. I think maybe the DOT3 I used had a color change due to water absorption because the fluid was formulated to change colors to indicate water saturation and time to change. I'm not sure though. The fluid in the master cylinder was honey colored and a little dirty.

[MikeB]

Lots of discussions on brake fluid by the geeks (like we aren't?) on the BobIsTheOilGuy.com forum. Dot 3 is the way to go for our trucks. And it seems there is little to no difference among brands. I just bought 2 quarts at a NAPA store for $8.99 each.