Brake fluid
 

New brake system...front to back...left to right... on a 70 GMC. 4 drums if it matters. Im an old guy, always used Dot-3. Went to my local auto parts store. No plain old Dot-3. For all intents, only synthetics. Didn't buy any. Hope someone can educate me on the new stuff.

Thanks much...Don.

Re: Brake fluid
 

This motul is my go to:
https://www.amazon.com/gp/product/B07KYWH2W8/

I used to use a Bosch 5.1 but my friends with track cars prefer the Motul...

Re: Brake fluid
 

I just bought a quart of DOT-3 last Friday at WM.

Re: Brake fluid
 

I bought DOT 3 yesterday at Auto Zone. They've got DOT 4 also.

Re: Brake fluid
 

It's your parts store . I bought a quart of DOT 3 today at NAPA.

But to your question this should help understand the differences.

https://www.machinerylubrication.com...om%20the%20air.

https://en.m.wikipedia.org/wiki/Brake_fluid

Re: Brake fluid
 

I used the O'Reilly Parts Store brand of DOT3 and it says synthetic on the label. It did not sway me one way or the other regarding choosing it to buy. If it says DOT3 it meets the required specifications and is suitable. I think it should be completely flushed out and replaced every couple of years just like coolant.

Re: Brake fluid
 

Is there a Tractor Supply nearby? This is what I have been using. DOT 3, not sure if it is synthetic or not, probably. I have had a hard time recently finding non-synthetic oils, so it seems I may be switching everything over soon, as I am not paying the same or more for a non-synthetic oil. $7 a quart.....same price at WalMart it seems.

https://www.tractorsupply.com/tsc/pr...fluid-32-fl-oz

Re: Brake fluid
 

All the DOT3 at O'Reilly was synthetic

Re: Brake fluid
 

Anecdotally, it's all been synthetic for a long while:
https://bobistheoilguy.com/forums/th...thetic.158663/

Castrol and Valvoline make a DOT3&DOT4 if that helps

Re: Brake fluid
 

Store did have bottles of dot-3/dot-4. Wasn't sure it was what I wanted.

Thanks all...much appreciated...Don.

Re: Brake fluid
 

Yup people confuse synthetic as being silicon fluid they are two different animals any Dot 3 will work in our trucks

Re: Brake fluid
 

I thought Dot 5 fluids were silicon. But here we have Motul non-silicon Dot 5.1 that can't be mixed with Dot 5. Apparently its properties exceed those of Dot 3 and Dot 4.

Thanks for the tip, because I am getting ready to 100% bleed my brakes, even though it's only been 10-15 years.

And, now for something completely confusing...

Re: Brake fluid
 

Oh yeah, how much fluid should I need to completely flush the system? 69 C10, GM square body discs, rear drums.

Re: Brake fluid
 

No need to rush into that. ;-)

Re: Brake fluid
 

I'd likely buy a couple qts, maybe 3 to be safe, and it is always good to have some around, just keep in sealed container, as it should not go bad, until exposed to moisture in the air. Brake fluid attracts moisture if not sealed. Or at least that used to be the case.

Re: Brake fluid
 

I searched this site for the same question.
Most all said 2 quarts.
I bought 3 for my project just in case.

HTH...Don.

Re: Brake fluid
 

Thanks, guys.

As they say in Texas, I'm fixin' to start thinkin' about doin' it.

Re: Brake fluid
 

One of the guys over on BobIsTheOilGuy said this when I asked about using DOT 5.1:

"I’m kinda hesitant using a DOT 4LV/5.1(ATE SL6/Super DOT 5.1, Bosch ESI6 or any of the OE-spec LV fluids) if your brake hydraulics aren’t all new, you never know of the lower viscosity fluid might seep out of old seals."

Others also recommened AC Delco or Motorcraft DOT 3.

Re: Brake fluid
 

Yes.

Parts that had Dot 3 running through them take Dot 3, though I believe Dot 4 will mix with them. Never mix DOT 5 or 5.1 with 3 or 4 simply because they don't mix. Downside of 3 and 4 are leaks that haven't been cleaned right away will damage all finishes.

DOT 5 has it's fans, but after watching a video of someone running tests, to my knowledge it's not correct for the old stuff because the silicone expands and distorts the rubber.

Re: Brake fluid
 

DOT 5.1 is glycol based and can be mixed with DOT 3 and DOT 4, although there is no need to mix any of these fluids with each other or think about mixing the fluids. It is easy and cheap to just mostly empty the reservoir in the master cylinder and put fresh of whatever fluid you want to use, and bleed about a quart of it through the system thoroughly to complete replace and refresh. So a newly-acquired truck where you don't know what fluid is in there should just be flushed and put the good ol' DOT 3 (labeled synthetic or not) in it and be done. If you ever have a leak or need to replace a part such as a cylinder or caliper or tubing, or even brake pads or shoes, it's time to repeat the full flush and change on the whole system, no reason not to do it. On a 67-72 truck there is no need to consider using anything other than DOT 3. If it's kept fresh, it's not going to boil or break down or give any problems. Just my opinion.

Re: Brake fluid
 

You are confusing DOT 5 with DOT 5.1. DOT 5.1 has no problem mixing with DOT 3 or 4. DOT 5 is silicone based and cannot be mixed with anything else.

Re: Brake fluid
 

Thank you