The DOT 5 brake fluid conundrum.
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Well yesterday I installed my Viton Oringed caliper. Brakes bleed up fine.
Car is still on stands. 24 hours plus later. No brake fluid leaks. Any luck with the weather I'll get it down off the stands and test drive this weekend.
Car is still on stands. 24 hours plus later. No brake fluid leaks. Any luck with the weather I'll get it down off the stands and test drive this weekend.
Le Mans Master
Joined: Oct 2005
Posts: 6,247
Likes: 471
From: Glen Allen, VA
Lip seal calipers on mine and DOT 5 fluid. I've owned mine for 17.5 years and put about 40K miles on it. I have put 2 calipers on it in that time. Did DOT 5 cause my problem with either one? I have no idea, but I have no intention of flushing out the DOT 5. It was in the car when I bought it, and a month ago with my latest caliper replacement I didn't even consider replacing the DOT 5. Bright purple fluid to the RR caliper (the one I replaced) and was happy. I did have to bleed the brakes a couple of times to get a good pedal, but they work great now, as they did before the caliper needed replacing.
3rd Gear
Joined: May 2025
Posts: 3
Likes: 0
OK, let's start off with stating the obvious. There have been a whole lot of threads as of late concerning brakes. And the topic of DOT 5 fluid and the warnings on Corvette brake products not to use it.
Not long ago I put up a thread concerning a supposedly new caliper that came to me in poorly rebuilt condition. In that thread I posted up a photo of a O ring that came in a kit I purchased in a attempt to repair this garbage I paid good money for.
this O ring convertion kit came with what I feel where some really crappy looking O rings.
It was brought to light by a fellow former member that in the early days of these O ring convertions that the slight mould seam on the O ring would wear and then cause leaks. So the Corvette aftermarket sought out O rings with the seam offset.
My research seems to show that this is the approximate time the DOT 5 warnings came into being.
It seems that one rubber compound seems to work well with both DOT 3 / 4 and DOT 5. This is the EPDM rubber compound commonly used in brake parts throughout the automotive industry for approximately 50 years.
However, when special O rings where manufactured specifically for Corvette calipers. This is not the rubber compound that was used. Thus this crappy looking O ring that doesn't like DOT 5.
the rubber they use does play nice with DOT 3 and 4. But doesn't like DOT 5.
So I go into a major bearing and seal shop armed with a crappy O ring and ask them to get me a dozen of these in EPDM rubber. The guy looked through every book he had.
No dice. No one made a O ring in this size in this type of compound.
I am starting to see the problem here.
.
So, I says what compounds of rubber can you get this size O ring in?
the answer was 2 . One wasn't useful for a moving wear part whatsoever. The other was Viton. I know the Viton name as it is common with engine seals.
So, I did some more research.
Seams Viton isn't used in brake systems inspite of its good wear qualities in moving applications because it doesn't play well with DOT 3 and 4 brake fluid.
However, Viton plays very, very nice with silicone based fluids.
In fact the compound of choice with silicone based fluids.
So, I now have 4 , part number BS318V O rings that fit the pistons of a rear O ring conversion kit.
Now, using these O rings will absolutely not work with DOT 4. But according to everything I have read. Should work fine with DOT 5.
Now of course this is just for a rear caliper.
And this is a ongoing experiment. But I am fairly certain that running a Viton O ring in place of the supplied O rings with a O ring convertion kit should solve the problem.
As long as one doesn't change back to hydroscopic brake fluid.
Thought I'd open this up to discussion.
Me, I'm going to build one rear caliper and give it a trial.
Cause they are NOT going to make me change brake fluid.
(My parents didn't raise sheep).
Not long ago I put up a thread concerning a supposedly new caliper that came to me in poorly rebuilt condition. In that thread I posted up a photo of a O ring that came in a kit I purchased in a attempt to repair this garbage I paid good money for.
this O ring convertion kit came with what I feel where some really crappy looking O rings.
It was brought to light by a fellow former member that in the early days of these O ring convertions that the slight mould seam on the O ring would wear and then cause leaks. So the Corvette aftermarket sought out O rings with the seam offset.
My research seems to show that this is the approximate time the DOT 5 warnings came into being.
It seems that one rubber compound seems to work well with both DOT 3 / 4 and DOT 5. This is the EPDM rubber compound commonly used in brake parts throughout the automotive industry for approximately 50 years.
However, when special O rings where manufactured specifically for Corvette calipers. This is not the rubber compound that was used. Thus this crappy looking O ring that doesn't like DOT 5.
the rubber they use does play nice with DOT 3 and 4. But doesn't like DOT 5.
So I go into a major bearing and seal shop armed with a crappy O ring and ask them to get me a dozen of these in EPDM rubber. The guy looked through every book he had.
No dice. No one made a O ring in this size in this type of compound.
I am starting to see the problem here.
.
So, I says what compounds of rubber can you get this size O ring in?
the answer was 2 . One wasn't useful for a moving wear part whatsoever. The other was Viton. I know the Viton name as it is common with engine seals.
So, I did some more research.
Seams Viton isn't used in brake systems inspite of its good wear qualities in moving applications because it doesn't play well with DOT 3 and 4 brake fluid.
However, Viton plays very, very nice with silicone based fluids.
In fact the compound of choice with silicone based fluids.
So, I now have 4 , part number BS318V O rings that fit the pistons of a rear O ring conversion kit.
Now, using these O rings will absolutely not work with DOT 4. But according to everything I have read. Should work fine with DOT 5.
Now of course this is just for a rear caliper.
And this is a ongoing experiment. But I am fairly certain that running a Viton O ring in place of the supplied O rings with a O ring convertion kit should solve the problem.
As long as one doesn't change back to hydroscopic brake fluid.
Thought I'd open this up to discussion.
Me, I'm going to build one rear caliper and give it a trial.
Cause they are NOT going to make me change brake fluid.
(My parents didn't raise sheep).
https://www.theoringstore.com/store/...cPath=368_2278
Instructor
Joined: Jan 2019
Posts: 167
Likes: 33
From: Connecticut, USA
I was reading through these messages, and unless I missed it, nobody stated the obvious. DOT 5 is silicone, and not compatible with anything petroleum based...DOT 3, 4. Mix the two and you could ruin your brakes. DOT 5 because it is silicone, it does not absorb water. It is used in my Harley-Davidson. I don't think once you use DOT 3,4 you can switch to DOT 5 because you can't get it all out of the system. The difference between DOT 3 and DOT 4 is DOT 4 has a higher boiling point.
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Well, truth is, the two just don't mix. You can't mix the two. No they won't mix and form some kind of monster. That's total nonsense.
Harley stopped using DOT 5 from the factory about 20 years ago. So ,.... We have converted numerous newer Harley's to dot5. By just simply doing a standard brake fluid flush.
Absolutely none of them had there brakes ruined.
I converted my Corvette over to DOT 5 back in the 90's.
I'm still driving it. And believe it or not. It didn't ruin my brakes.
The ONLY issue is a certain company that supplies O Ring caliper parts for our C2 and C3 Corvette's decided that the parting seam on a standard O ring being right on the outer edge could possibly cause an issue.
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
However, the main reason I wanted to jump back in here is to try to keep things factual. Hard to say or guess how many hundreds of thousands of vehicles have been converted over to DOT 5 brake fluid. I know it's extremely popular in the old car hobby in general with seldom used cars. And also extremely popular with motorcycles which are also seldom used.
Add into that the boiling point of DOT 5 is higher still than Dot4. And with all those hundreds of thousands of vehicles out there, I personally have never heard of anyone's brakes being ruined. I have heard nonsense rumours. But never once seen anything of the sort.
Of course as a professional mechanic working 6 days a week for 50 years on DOT 5 equipped vehicles, what do I know.
Harley stopped using DOT 5 from the factory about 20 years ago. So ,.... We have converted numerous newer Harley's to dot5. By just simply doing a standard brake fluid flush.
Absolutely none of them had there brakes ruined.
I converted my Corvette over to DOT 5 back in the 90's.
I'm still driving it. And believe it or not. It didn't ruin my brakes.
The ONLY issue is a certain company that supplies O Ring caliper parts for our C2 and C3 Corvette's decided that the parting seam on a standard O ring being right on the outer edge could possibly cause an issue.
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
However, the main reason I wanted to jump back in here is to try to keep things factual. Hard to say or guess how many hundreds of thousands of vehicles have been converted over to DOT 5 brake fluid. I know it's extremely popular in the old car hobby in general with seldom used cars. And also extremely popular with motorcycles which are also seldom used.
Add into that the boiling point of DOT 5 is higher still than Dot4. And with all those hundreds of thousands of vehicles out there, I personally have never heard of anyone's brakes being ruined. I have heard nonsense rumours. But never once seen anything of the sort.
Of course as a professional mechanic working 6 days a week for 50 years on DOT 5 equipped vehicles, what do I know.
Last edited by 4-vettes; May 29, 2025 at 04:24 AM.
Safety Car
Joined: May 2001
Posts: 4,759
Likes: 1,647
From: Fairfax Virginia
When I first rebuilt my four calipers I switched to the O-ring type seal and had great luck with them. After rebuilding the system I then put in the DOT 5 brake fluid and drove it for a few months. The braking system seemed to need to be "warmed" up before the pedal feel was the same. The brake pedal was different when the system was cool.
The switch from DOT 3 or 4 to DOT 5 was more than just a "simple fluid change". It changed the way the brakes feel while driving them. After a period of time I went and dumped the DOT 5 brake fluid in favor of DOT 4. After releasing the DOT 5 I went and ran through several quarts of some type of alcohol that was sold to me for this purpose flushing the lines and calipers. Then I filled the system with the DOT 4 and bled the brakes, fortunately for me, the brakes came back to the normal feeling system I was used to.
My 1984 Yamaha RZ 350 also has DOT 5 in the triple discs on my 350cc monster.... It also has EVANS NPG in the radiator...
The switch from DOT 3 or 4 to DOT 5 was more than just a "simple fluid change". It changed the way the brakes feel while driving them. After a period of time I went and dumped the DOT 5 brake fluid in favor of DOT 4. After releasing the DOT 5 I went and ran through several quarts of some type of alcohol that was sold to me for this purpose flushing the lines and calipers. Then I filled the system with the DOT 4 and bled the brakes, fortunately for me, the brakes came back to the normal feeling system I was used to.
My 1984 Yamaha RZ 350 also has DOT 5 in the triple discs on my 350cc monster.... It also has EVANS NPG in the radiator...
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I've seen you comment on this forum before about a different "Feel" in your brakes at different temps with Dot5.
personally. I think that's all in your head.
personally. I think that's all in your head.
Instructor
Joined: Apr 2024
Posts: 152
Likes: 36
From: Katy, Texas
Perhaps DOT 5 fluid has a propensity to entrain micro-air bubbles if the hydraulic system is not bled thoroughly, resulting in a different brake pedal feel compared to glycol-based fluids.
Jason
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
3rd Gear
Joined: May 2025
Posts: 3
Likes: 0
I was reading through these messages, and unless I missed it, nobody stated the obvious. DOT 5 is silicone, and not compatible with anything petroleum based...DOT 3, 4. Mix the two and you could ruin your brakes. DOT 5 because it is silicone, it does not absorb water. It is used in my Harley-Davidson. I don't think once you use DOT 3,4 you can switch to DOT 5 because you can't get it all out of the system. The difference between DOT 3 and DOT 4 is DOT 4 has a higher boiling point.
You are done when you get pure purple fluid at each corner with no air bubbles.
Instructor
Joined: Jan 2019
Posts: 167
Likes: 33
From: Connecticut, USA
The ONLY issue is a certain company that supplies O Ring caliper parts for our C2 and C3 Corvette's decided that the parting seam on a standard O ring being right on the outer edge could possibly cause an issue.
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
.
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
.
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
That's 2 years ago.
My car still stops hard and straight and no leaks.
And yes, DOT 5 brake fluid.
And I've run DOT 5 in my Corvette for about 30 years now all up. On the street and a few track days. Hot, cold, whatever. I find it performs perfectly. I would say brake pads can be sensitive to heat. But DOT 5 has a much greater usable temperature range than DOT4.
3rd Gear
Joined: May 2025
Posts: 3
Likes: 0
The ONLY issue is a certain company that supplies O Ring caliper parts for our C2 and C3 Corvette's decided that the parting seam on a standard O ring being right on the outer edge could possibly cause an issue.
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
So they have special O rings made that have the seam, / Parting line on a 30 degree (approximately) angle instead of the standard centered location.
Having these O rings made overseas, and to a price. They use a rubber that is fine for DOT 4, but not 5 as using the good rubber perhaps costs more?
I've never seen a good o-ring with any "parting seam." Where do you get these cheap orings with a parting seam on the OD ?
Dorman says that their rebuild kits for brake calipers are made from EPDM rubber and compatible with Dot 5 fluid.
I pulled up their kit for 1st gen Camaros and 65-82 Corvettes and there was nothing in the specs that said they were EPDM, so I emailed their tech support and explained the issue and asked if they could verify that their kits are EPDM and ok with Dot 5.
Hopefully they will get back to me soon and i'll let everyone know.
Then the question : If Dorman has rebuild kits ok with Dot 5, what excuse will these Corvette vendors use now ?
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,147
Likes: 4,258
From: US-PNW
DOT 5 is silicone, and not compatible with anything petroleum based...DOT 3, 4. Mix the two and you could ruin your brakes. DOT 5 because it is silicone, it does not absorb water. It is used in my Harley-Davidson. I don't think once you use DOT 3,4 you can switch to DOT 5 because you can't get it all out of the system. The difference between DOT 3 and DOT 4 is DOT 4 has a higher boiling point.
As others have replied, silicone 5 and 3,4,5.1 will not mix in the slightest. They just don't react with each other, so any remaining old fluid will just end up being pushed out during flushing.
Le Mans Master
Joined: Mar 2004
Posts: 7,503
Likes: 1,511
From: Little Rock AR
I saw the Dorman master cylinder rebuild kit at Rock Auto. It says EDPM in the description AND it includes BOTH pistons. In the past, nobody else had the front piston as far as I know.
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Good to know
7th Gear
Joined: Apr 2026
Posts: 7
Likes: 2
I've been dealing with a leak in a 66 Kelsey Hayes proportioning valve using DOT 5. The prop valve was rebuilt by White Post and they don't warranty when using DOT 5. If you're familiar with this prop valve, there is a 1" o-ring in the back held in place with a plate and c clip. A few years after the rebuild I noticed a little fluid had leaked out over the winter. I pulled the valve apart and bought a few different size EPDM orings and settled on the 119. Everything was fine for about 5 years and this past winter the leak reappeared. I had to buy the orings in bulk so I had lots of extras to experiment with.
The new o-ring had a cross section of 0.1070"
The leaky o-ring I removed had a cross section of 0.0960" (started at 0.1070")
So did it compress over time or did the DOT 5 shrink it? I need the o-ring to not shrink and if it swells a little that's fine based on its location and purpose.
Here's some interesting test results:
#1 - soaked with Blaster silicone spray and swelled from 0.1070" to 0.1285" I later dropped it into DOT 5 fluid and it shrunk back to 0.1185" and after a few weeks 0.1065"
#2 - soaked in motor oil and it swelled to 0.1180". I later dropped it into DOT5 and it shrunk to 0.1130" and after a few weeks stayed there
#3 - soaked in DOT 3 and it stayed at 0.1070" for weeks
#4 - soaked in DOT 5 and has gradually shrunk over a month from 0.1070" to 0.1020"
#5 - dropped the DOT 3 oring into DOT5 tonight to see if shrinks or retains the original cross section
For my particular use case, I need something that doesn't shrink with DOT5 over time and my guess is that is one of the reasons places like CSSB and White Post don't warranty parts used with DOT5
For what it's worth, the entire brake system was re-done 10 years ago with DOT5 in mind and there have been no other leaks
The new o-ring had a cross section of 0.1070"
The leaky o-ring I removed had a cross section of 0.0960" (started at 0.1070")
So did it compress over time or did the DOT 5 shrink it? I need the o-ring to not shrink and if it swells a little that's fine based on its location and purpose.
Here's some interesting test results:
#1 - soaked with Blaster silicone spray and swelled from 0.1070" to 0.1285" I later dropped it into DOT 5 fluid and it shrunk back to 0.1185" and after a few weeks 0.1065"
#2 - soaked in motor oil and it swelled to 0.1180". I later dropped it into DOT5 and it shrunk to 0.1130" and after a few weeks stayed there
#3 - soaked in DOT 3 and it stayed at 0.1070" for weeks
#4 - soaked in DOT 5 and has gradually shrunk over a month from 0.1070" to 0.1020"
#5 - dropped the DOT 3 oring into DOT5 tonight to see if shrinks or retains the original cross section
For my particular use case, I need something that doesn't shrink with DOT5 over time and my guess is that is one of the reasons places like CSSB and White Post don't warranty parts used with DOT5
For what it's worth, the entire brake system was re-done 10 years ago with DOT5 in mind and there have been no other leaks
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Have you tried a Viton O ring?
7th Gear
Joined: Apr 2026
Posts: 7
Likes: 2
Not yet, actually looking into that now.
Any opinion on which material, FEP or non-FEP? https://www.mcmaster.com/products/vi...stomer-o-rings
Any opinion on which material, FEP or non-FEP? https://www.mcmaster.com/products/vi...stomer-o-rings
Last edited by sg66; Apr 10, 2026 at 10:37 PM.
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,862
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Not a clue. Perhaps a question for the supplier.