Correct brake fluid - The 1947 - Present Chevrolet & GMC Truck Message Board Network

OUbesto · 08-08-2023, 12:44 PM

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** · 08-08-2023, 01:00 PM

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

OUbesto · 08-08-2023, 01:16 PM

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** · 08-08-2023, 01:51 PM

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

**Rust_never_sleeps** · 08-08-2023, 01:55 PM

Quote:

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 · 08-08-2023, 02:22 PM

Good stuff! Thanks

**71CHEVYSHORTBED402** · 08-09-2023, 02:17 AM

Quote:

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** · 08-09-2023, 09:11 AM

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** · 08-09-2023, 05:52 PM

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** · 08-09-2023, 06:30 PM

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** · 08-10-2023, 11:29 AM

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.

08-08-2023, 12:44 PM	#1
OUbesto Registered User Join Date: Oct 2015 Location: Lincolnshire, IL / Burlington, WI Posts: 209	Correct brake fluid 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. __________________ 1970 C-10 CST, LWB, 402/400, A/C

08-08-2023, 01:00 PM	#2
71CHEVYSHORTBED402 Senior Member Join Date: Feb 2009 Location: Northern Nevada Posts: 7,129	Re: Correct brake fluid DOT3, assuming he didn't replace the entire brake system and used DOT5. __________________ Tony 71 Custom Deluxe, SWB, 2WD, 402, A/C. I developed an assm. guide "kit" for restoring it from ground up. With assys, the guide accts for 1000s details such as OEM identifications& part numbers, written in short order. 700+ images include assm, illust., charts, and points of interest. Much of the info. applies to all 67-72 GM trucks, and to a lessor degree all 67-72 GM vehicles. My build thread, and more on the guide https://67-72chevytrucks.com/vboard/...d.php?t=730025

08-08-2023, 01:16 PM	#3
OUbesto Registered User Join Date: Oct 2015 Location: Lincolnshire, IL / Burlington, WI Posts: 209	Re: Correct brake fluid 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? __________________ 1970 C-10 CST, LWB, 402/400, A/C

08-08-2023, 01:51 PM	#4
MySons68C20 Senior Member Join Date: Sep 2014 Location: Wenatchee, Wa Posts: 899	Re: Correct brake fluid https://blog.amsoil.com/dot-3-and-do...he-difference/

08-08-2023, 02:22 PM	#6
OUbesto Registered User Join Date: Oct 2015 Location: Lincolnshire, IL / Burlington, WI Posts: 209	Re: Correct brake fluid Good stuff! Thanks __________________ 1970 C-10 CST, LWB, 402/400, A/C

08-09-2023, 09:11 AM	#8
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 3,567	Re: Correct brake fluid 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.

08-09-2023, 05:52 PM	#9
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 46,670	Re: Correct brake fluid 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. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!

08-09-2023, 06:30 PM	#10
dmjlambert Senior Member Join Date: Apr 2016 Location: Cypress, TX Posts: 3,567	Re: Correct brake fluid 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.

08-10-2023, 11:29 AM	#11
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,429	Re: Correct brake fluid 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. __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then! Last edited by MikeB; 08-13-2023 at 11:28 AM.



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