jzwu

I now have DOT 3 brake fluid as it came from factory in the car. Today I just opened a bottle of DOT 4 brake fluid to change for my other car and it still has a lot left.

Thinking about changing my SC430 to DOT 4 too. I think I can mix DOT 3 and 4 fluid but is there any downside of mixing them?

Anyone is using DOT 4 fluid? I guess there is no difference for street driving.

Pnuge88

iTrader: (1)

factory is best. dot 3 sir. can't remember if dot 4 or 5 was a silicone base which doesn't absorb moisture or something like that but would need to be changed more often or else it can corrode your brake lines. i forget why it would corrode if it doesn't absorb moisture. something about super dupe microscopic bubbles. just had a whole test on this too lol

jzwu

DOT 5 is silicone based and it doesn't absorb water so you need to do a complete flush. Otherwise, there will be water bubbles in the system.

DOT 4 does have a few benefits over DOT 3 like higher boiling points dry or wet and slower water absorption. But unless you do tracking or competition driving the brake fluid will probably never reach the boiling point during street driving.

VVTiBob

DOT 3 fluids are based on glycol and glycol esters, DOT 4 fluids are very similar, but contain borate esters too. It's those borate esters which offer more anti corrosive properties, especially when water eventually invades. If it were me, I'd flush, not bleed the whole system, rather than mix the two.
I'd also disagree regarding reaching boiling points on street driving. It really depends on your driving/braking habits. Heat generated by brakes can rapidly reach higher temperatures 1k+ and is quickly transferred to the calipers. While DOT 4 offers more protections from boiling, it's only about a 20° difference between the two boiling points ("wet" older fluid with water contamination). New fresh "dry" fluid has a substantially higher boiling point.

If your system hasn't been completely and correctly flushed in years, it's probably time to do it anyway.

jzwu

OK. What's the difference between flushing and bleeding the system? You said to flush but not bleed.

A1Lexis

iTrader: (6)

Bleeding just gets the little air bubbles out and flushing completely replaces the fluid.

To flush:

Drain your brake fluid resevoir with a turkey baster (as much as possible). Then use a Motive brake bleeder, put 2 Quarts of new brake fluid in it, hook it up, pump it to proper psi. Attach your bleeder tube to bleeder valve on caliper, and let it drain until the new stuff starts coming out.

VVTiBob

Bleeding will NOT remove ALL the water which has accumulated over time. By flushing, you remove all contaminants with a fresh infusion of new untainted brake fluid. The term WET boiling temperature usually refers to a water saturation around 4%. It's the water that has a corrosive affect on the brake system, and, in a small way, reduces braking efficiency. The question you really need to answer is when was the last system flush.

harleydjce

Stick with DOT 3. DOT 4 is usually not compatible with DOT 3 systems. Super DOT 4 is but Lexus (at least in Australia) advise against it.

jzwu

I did more research. You are right that DOT 4 is often not good for a brake system designed for DOT 3 due to the different chemical composition.

kjcole

Huh?

According to the brake fluid chemists DOT 3 and DOT 4 are virtually interchangeable, and mixable (miscible). Toyota is the only manufacturer that says not to use DOT 4 (DOT 3 only), and that rather famous position is controversial and has been widely questioned since there is no scientific or technical basis for it - unless you are using purely organic pads, then apparently DOT 3 is recommended. Brake component manufacturers (who have to warranty their parts) who make components designed for non-silicone fluid systems (that is, not DOT 5) state that DOT 3 and DOT 4 are both compatible and can be mixed. I've run DOT 4 in my cars and motorcycles calling for DOT 3 for years with no problems. I've never had to rebuild any of those calipers or master cylinders after years of use and hundreds of thousands of miles.

I'm running DOT 4 in my 2002 - so I'll let you know if there's a failure (or my widow and orphans will).

jzwu

Hahaha, you are being too serious. I know DOT 3 & 4 are mixable and in most cases interchangeable.

However, DOT 4 fluid does have a different chemical composition, such as added borate. Some of the chemicals may not be best for the seals in a DOT 3 brake system. That is not saying the system will fail.

Another fact is DOT 4 fluid has a higher boiling point and absorbs water more slowly. But once it starts to absorb water the boiling point drops more quickly than DOT 3 fluid. The curve is steeper, which results in a more pronounced deteriorating braking effect. I think that's why cars using DOT 4 fluid requires flushing the brake system every two years.

kjcole

Yeah, I have a problem taking things seriously (I'm a university professor and scientist so I have a hard time turning that off when technical/engineering discussions pop up!). Your points on the relative merits of DOT3 and 4 are well balanced. I usually change my brake fluid every few years anyway, or at least when it is showing visible darkening. I don't think that there is reason to fret too much over the DOT 3/4 issue, just so people understand when and when not to go with silicone based fluid.

darbar

I'm getting ready ot change mine as well. I think to be on the safe side (Lexus warranty), I'll go to the dealer and purchase DOT3 from them and flush it myself. Good thread above.

jzwu

Any decent DOT 3 fluid is good. I bet Lexus dealer will sell you the brake fluid like sacred water.

harleydjce

I use AMSoil Dot 3. No issues from my Dealer whatsoever but they won't put any DOT 4 or Super DOT 4 or DOT5 fluid in.