Brake Fluid
 

Anyone know what type of brake fluid is in the car from the factory? The manual states that either DOT 3 or 4 can be used, but advises not to mix both. Information about brake fluid is all over the map with fact and fiction blended together. Focusing just on DOT 3 and 4 here, the basic difference is that Dot 4 adds Borate to the mix, has a higher boiling point and a lower viscosity that lends itself to better performance in traction control systems. I am one of those who diligently keeps after the brake fluid especially being in a wet climate flushing the brake system every two years or three years at most. Brake fade on a long downgrade when downshifting isn’t enough is good motivation. One odd fact is while DOT 4 has a higher boiling point, it takes less water absorption over time so the boiling point drops faster as a percentage than DOT 3. The advice goes on to say that if you are using DOT 4, the fluid replacement should be done on a shorter interval. All this is nit-picky and I am reminded by what an old boss said about getting caught up in the details, “Don’t be picking the fly shit out of the pepper.” One source did say that you can add DOT 4 to DOT 3 if necessary, but not the other way around. My Ascent is due for a brake fluid change, but no rush as it is low miles and kept in a garage, so the exposure is water is not high. Just want to be consistent with what I use. A friend on advice from a know-it-all added a significant amount of a silicone based brake fluid to top off the reservoir in his Toyota. Then strange things started to happen. Complete system flush fixed that. Don’t want to go down that road.

DOT3, DOT4, and DOT5.1 are all generally compatible within a braking system, as all are all glycol-based brake fluids.


DOT4 & DOT5.1 tends to have higher temperature resistance than DOT3 for performance driving. However that temperature resistance comes with the price of higher hygroscopy - they tend to absorb more water from air compared to DOT3. This essentially degrades their performance more quickly and means they need to be changed more often to maintain their characteristics.

--Most people who use high performance fluids in HPDE/race settings don't encounter this issue as they change the fluid more often than the standard service interval just due to the rigors and demands on a road course


Generally avoiding mixing has less to do with straight compatibility/damage to the system. I think it has to do with maintaining the predictability of the braking system over time, as the performance level and drop-off of the types of fluids change at different rates. Starting from DOT4 and mixing in DOT3 means you're lowering the effective boiling point. If a driver is expecting DOT4 performance, they are at risk of an incident if it is not delivered.

Is this thread for real? This is a joke, right?

What makes you think this thread is a joke? Do you mean, "How complicated can it be? After all its only brake fluid not rocket science." I once had a car so old that it had castor oil in the brake system. My first car had a master cylinder with one brake line that branched off to the four wheels. A problem anywhere in the system and your foot goes straight to the floor. Then came dual braking systems that had front/rear or diagonal systems. The next step in the evolution was ABS. I loved that one; your stopping distance increased but at least the car stayed straight so you could rear-end the car in front of you dead on. Now we have vehicle stability and traction control added to the mix. The reason I started this thread is simple: the manual states that you can use DOT 3 or DOT 4 and mixing types or even different brands is not recommended. But, it does not say what brake fluid is in the car from the factory. Here is some light reading that explains that there is a little more to this subject. The part about viscosity at low temperatures is interesting and maybe relevant if you drive an AWD Subaru in the winter.

https://www.remmenbrakes.com/brake-f...d-know-part-1/
https://www.remmenbrakes.com/brake-fluid-standards/

Flush the system with whatever you want. Then you know what’s in it.

Exactly. A neighbor hauls a backhoe on a flatbed behind his big truck and if it wasn’t for the trailer brakes, he would have lost the whole mess. After giving him the third degree treatment, I find out that the front calipers were replaced and the shade tree in South Dakota had bled the system with whatever brake fluid he had on hand. Best guess is the jug was leftover from when Ronald Reagan was President. Complete flush with DOT 4 restored things to normal. Knowing someone with a big flatbed is great when you want to move cars on the cheap.