# Can I mix DOT3 and DOT4 brake fluid **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** mercury, fluids, brakes\ **Created:** [February 1, 2010, 4:55pm UTC](https://community.cartalk.com/t/can-i-mix-dot3-and-dot4-brake-fluid/43629 "2010-02-01T16:55:06Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![michaelm\_1958](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@michaelm\_1958](https://community.cartalk.com/u/michaelm_1958)\ **Post date:** [February 1, 2010, 4:55pm UTC](https://community.cartalk.com/t/can-i-mix-dot3-and-dot4-brake-fluid/43629/1 "2010-02-01T16:55:06Z")
The idiot light for my brake system, not the ABS, kept coming on when I stopped hard so I thought the cheapest fix to try was to add some more fluid. My owner’s manual calls for DOT3 but when looking at the wide array of choices at WallyWorld I saw DOT4 had higher boil off specs than DOT3 and was about the same price, so I bought it and added it to my car. Afterward I thought I remembered something from the past about not mixing different types of brake fluids. \>\>\> So here are the questions: Did I mess up my brake system by adding synthetic DOT4 fluid to the manufacturer’s original DOT3, probably non-synthetic, fluid? Do I now need to get my system flushed? ---
**Author:** ![Caddyman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/caddyman/32/23_2.png) [@Caddyman](https://community.cartalk.com/u/Caddyman)\ **Post date:** [February 1, 2010, 4:58pm UTC](https://community.cartalk.com/t/can-i-mix-dot3-and-dot4-brake-fluid/43629/2 "2010-02-01T16:58:56Z")
DOT-3 and DOT-4 are compatible with each other, no problem. The difference between them is very small…More of a Shelf=Space issue then anything else… ---
**Author:** ![texases](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/texases/32/862_2.png) [@texases](https://community.cartalk.com/u/texases)\ **Post date:** [February 1, 2010, 5:12pm UTC](https://community.cartalk.com/t/can-i-mix-dot3-and-dot4-brake-fluid/43629/3 "2010-02-01T17:12:39Z")
Yep, no problem. DOT4 is different than the silicone-based and incompatible DOT5 fluid, I don’t know if DOT4 is ‘synthetic’, but it’s compatible with DOT3. ---
**Author:** ![jtsanders](https://avatars.discourse-cdn.com/v4/letter/j/c2a13f/32.png) [@jtsanders](https://community.cartalk.com/u/jtsanders)\ **Post date:** [February 2, 2010, 1:35am UTC](https://community.cartalk.com/t/can-i-mix-dot3-and-dot4-brake-fluid/43629/4 "2010-02-02T01:35:25Z")
You’re fine. But now that the bottle is open, any remaining unused brake fluid will be subject to water absorption. Make sure the top is closed well if you plan to use it again.