Add new brake fluid to existing fluid?
 

I'm doing a track day on Monday and was checking out my brake fluid. Looks a little low and hasn't been changed in almost 2 years. Can I add Motul DOT 4 to existing factory brake fluid ? I"m guessing no but thought I'd give it a shot. I know I need to change it all but won't have time before Monday.

You can; it's compatible, but after 2 years your old fluid will have a lower boiling point and the splash of fresh Motul isn't going to change that much. Also, you will have unsealed a $20 bottle only to use maybe a quarter of it.

I recommend finding sometime in the next 3.5 days a spare hour to change the fluid. If you have a Tech 2 you can do the automated bleed. It does get all the old fluid out of the ABS block, but the real win is that it makes one-man bleeds super quick and easy.

Thanks for posting. One of my concerns was if it's DOT 3 in the car and I'm putting in DOT 4, is that an issue? There's no way I can get to changing it before Monday due to work.

I regularly suck the old oil in the reservoir once every 3 months and replace it fresh dot 3. After 6 years when I do a proper bleed the fluid that comes out is very clean. I am now going to do it every 10 years. Plus after you put in fresh oil if you do hard stops to activate the ABS, it will circulate the old oil.

I do not think you can mix 3 with 4. You have to empty all the fluid, flush the lines with dot 4 first.

Plus .4 detorietes faster than dot 3. It is more hydroscopic, lot more. I would not do it

dot 3 and dot 4 are compatible

https://blog.amsoil.com/dot-3-and-do...oiling%20point.

Bosch dot 5.1, great specs
https://www.rockauto.com/en/moreinfo...=11389&jsn=355

I do the same thing with both my brake fluid as well as my clutch fluid. --- but I refresh fluids every 6 months and the clutch fluid I have never completely changed since since 2003. AS far as my radiator coolant, I change just the overflow tank with fresh GM Dexcool every 3 years and do a complete radiator drain/flush every 10 years. All fluids are clean all the time.

Are you guys just using the Ranger method of sucking out brake fluid from the fill hole and then refilling? Does this still require bleeding the brakes?

Putting new fluid in the reservoir will make no difference. It will not get down to the caliper where it’s needed. The bleeders have to get opened so new fluid flows there.

Exactly.

Bleed the brakes. Your life and that of others on the track will thank you.

Beginners are the ones that usually boil the fluid or fade their pads on track. What you're proposing to do is just as bad as doing nothing. Remember this about brakes.....They work until they don't.

Lastly....You also do not need a Tech 2. There's virtually no fluid in the ABS system. It's not sufficient enough to dilute the fluid in the calipers. Using a MityVac or a Motive pressure bleeder makes this a one person job as well.

Yep. It works vey well for all. Steering fluid, brake fluid, clutch fluid I never tried it with coolant. I completely change it every 5 years as per the GM suggestion.

Remember....The OP is doing a TRACK day. The requirements are much different.

Yes of course totally different. I do not track mine ( l love the car so much that I do not abuse it, still drive very spirited, track takes a lot of abuse)

My car is a cruzer. I don't track my car or beat the piss out of my car every time I get in it. Yea, I goose it every once in awhile but that's it. I change my brake fluid myself every 6-8 years and my fluid comes out clear because of the ranger method I use for my clutch, brakes and coolant. There must be some circulation going on otherwise the brake fluid in the brake reservoir would always stay clean --- and it doesn't. It gets dirty as hell if you don't change it. Same for the clutch fluid. Works for me. Car stops fine and still the same calipers and master cylinder for 20+ years now.

Thanks for all the replies. I was going to just cancel the track day because of the brake fluid….then one of the shops local to me said they could get me in this morning. Total relief! He showed me the old fluid and it was completely black..definitely wouldn't have been safe.
Can’t wait for Monday!

If your going to run HPDE, then really want to make sure your going into the event with clean/dry fluid, and then do another fluid flush with new fluid after the event.

As the fluid started to reach is boiling point from HPDE, is really pulls in humidity from the air, and the fluid is not longer considered dry/clean by the end of that weekend event, and will have a lower boiling point.

As for what fluid, unless your running R compound tires or more of a advance driver on super cups, Preston dot 4 is $12 a quart (enough to do a completed fluid flush), and has a dry boiling point of 510* that most novices will not get the brake system to to begin with during a HPDE.

Very good! One less problem and worry. Enjoy the track!

I've been going to road course tracks since '06 and also enjoy the car work. If you want to learn to bleed brakes there is a ton of help here and videos. I think the worst is cracking the bleeders open if they have never been or been gorilla tightened.

Have fun at the track! I assume Watkins Glenn? Nowhere better to be on a Monday than at the track turning laps :)

--Aaron

Just got back from New York Safety Track (about 3 hours east of Watkins Glen). Car was amazing…my driving skills not so much. First time in 15 years on track and took me a while to get my bearings…. but had an absolute blast!
Had to cut my last run short as oil temps hit 260*
Before I this again I’ll need an oil cooler. Thanks for all the help fellas.