The DOT 5 brake fluid conundrum.
Race Director
Joined: Mar 2017
Posts: 11,960
Likes: 4,518
From: Virginia
O-rings are pretty cheap. Perhaps buy a few of the correct size, and repeat your experiment.
As @4-vettes suggested, Viton would be my suggestion, if I had to run DOT 5. Grok suggests EPDM, and sources a Corvette Forum posting. Even better, I'd suggest you carefully consider NOT using DOT 5.
https://www.corvetteforum.com/forums...id-please.html
As @4-vettes suggested, Viton would be my suggestion, if I had to run DOT 5. Grok suggests EPDM, and sources a Corvette Forum posting. Even better, I'd suggest you carefully consider NOT using DOT 5.
https://www.corvetteforum.com/forums...id-please.html
Racer
Joined: May 2020
Posts: 498
Likes: 167
I was experiencing fade when doing some “spirited” driving so I purchased a DOT 5.1 brake fluid. It is glycol based unlike DOT 5, but has a higher boiling point than DOT 4. I haven’t put it yet, because bleeding all the fluid out of the brakes will be a pain.
Last edited by JC 1975; Apr 11, 2026 at 02:45 PM.
Drifting
Joined: Jun 2025
Posts: 1,739
Likes: 605
Do suggest a flush, BUT DOT 5.1 (NOT simply 5) is completely compatible And miscible with either DOT 3 Or DOT 4.
7th Gear
Joined: Apr 2026
Posts: 7
Likes: 2
O-rings are pretty cheap. Perhaps buy a few of the correct size, and repeat your experiment.
As @4-vettes suggested, Viton would be my suggestion, if I had to run DOT 5. Grok suggests EPDM, and sources a Corvette Forum posting. Even better, I'd suggest you carefully consider NOT using DOT 5.
https://www.corvetteforum.com/forums...id-please.html
As @4-vettes suggested, Viton would be my suggestion, if I had to run DOT 5. Grok suggests EPDM, and sources a Corvette Forum posting. Even better, I'd suggest you carefully consider NOT using DOT 5.
https://www.corvetteforum.com/forums...id-please.html
I've been using DOT5 for close to 40 years and the only scenario I would 100% agree to not use it is for high altitudes. Back in the 80's I drove up the Mt Washington auto road and when I got to the top, the brake pedal went to the floor and had to be towed down the mountain. Once I got to the bottom the brakes were back to normal.
Something else I've learned from this is that the silicone in DOT5 does not react with EDPM the same way silicone lubricant does. One seems to shrink o-rings and the other makes them swell.
I'll be back in a few weeks with Viton findings
Last edited by sg66; Apr 13, 2026 at 10:51 AM.
7th Gear
Joined: Apr 2026
Posts: 7
Likes: 2
I've had a few weeks to do lots of research and compare Viton and EPDM and concluded that EPDM o-ring on the rear cap of the Kelsey Hayes prop valve failed over time for two reasons:
I don't know what their definition of compatible is but new EPDM o-rings shrank from 1.1540 to 1.1435 after 2 weeks, the same as the other fluid I had. Viton didn't change at all.
By the way, the EPDM o-ring I started soaking in DOT3 over a month ago hasn't shrunk at all.
So that's it, now I understand why many places don't recommend or warranty brake components when using DOT5
If you're interested in learning more about o-rings than you ever wanted to:
- DOT 5 causes EPDM to shrink. A new -119 o-ring has an O.D. of 1.1540" when installed on the rear cap. The leaking o-ring, 4-5 years old had shrunk to 1.1270" O.D. The cross section of the o-ring shrunk ~0.01". These don't sound like much but together they are enough to let fluid leak past the rear cap.
- EPDM "compression set" compounds the shrinkage problem. When compressed for a period of time, EPDM is more prone than Viton to retain its compressed state where Viton is more likely to bounce back. The o-ring in the prop valve is under constant pressure by the internal spring used for adjusting brake pressure so while it's shrinking, it's not bouncing back as much to fill the void.
I don't know what their definition of compatible is but new EPDM o-rings shrank from 1.1540 to 1.1435 after 2 weeks, the same as the other fluid I had. Viton didn't change at all.
By the way, the EPDM o-ring I started soaking in DOT3 over a month ago hasn't shrunk at all.
So that's it, now I understand why many places don't recommend or warranty brake components when using DOT5
If you're interested in learning more about o-rings than you ever wanted to:
Last edited by sg66; Apr 29, 2026 at 11:10 PM. Reason: Add attachment
Thread Starter
Race Director
Joined: Nov 2006
Posts: 13,282
Likes: 7,863
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
And yet my prop valve is original to the car. And I've been running DOT 5 for about 30 years. With zero leaks or issues.
7th Gear
Joined: Apr 2026
Posts: 7
Likes: 2
For what it's worth, my prop valve had been rebuilt with a fresh o-ring. Anyhow I'm feeling good about the Viton, thanks for the tip.
Looks like some similar testing happened over here https://www.corvetteforum.com/forums...ake-fluid.html
Last edited by sg66; Apr 30, 2026 at 08:07 AM. Reason: Add link
