the blur

I have seen many references to only using DOT 3 fluid. I brought a new master cylinder cap, and it says DOT 3 only. I have also read more than once, if it says use DOT 3, only use DOT 3 and not DOT 4, not DOT 5. Something about DOT 4 has difference additives, and DOT 5 is silicon based.

Having said all that, I use DOT 4, and have never had a problem. but now it got me thinking.

bogus

Use Valvoline Synpower Brake fluid. Great stuff!!!!!

Dartvader

Tony, of Tony's Corvettes says Dot 5 will mess up the antilock brake system. He is a very cautious, but highly regarded guy, and does only what he can guarantee. I certainly would feel OK about Bogus's recommendations, but if the question is "can I put dot5 in it?". I would not do it if it has antilock, skid control, or any system that applies and releases the brakes through an electronicly controled module of some kind. I have put dot5 in my older cars, and they seem to like it fine. It does not pick up moisture, so over the years is kinder to the inner workings of brake parts.

tkrussell

Short answer is that Dot 4 fluids are fine.

Dot 4 is "conventional" fluid, just like Dot 3, with somewhat different additives. Castrol SRF is Dot 4, for example. No one has ever questioned its performance in the Vette under the most extreme conditions.

All Dot 3 and Dot 4 are hygroscopic, that is, they absorb water vapor from the air, and over time will degrade if left exposed to atmosphere, even the air in the master cylinder. That is why Dot 3 and Dot 4 fluids should be changed "regularly". I do it once a year, but I am more cautious than most in a hiperf street car. Advantage is that any water in the system will be absorbed in the fluid, degrading performance slightly, but retaining a boiling point still well above 210 F of plain water.

Dot 5 fluid is Silicone based. It was developed and designed for high temp applications and archival storage, among other reasons. It is not miscible, or mixable, with Dot 3 and or 4 fluids, and it will not absorb water at all. Problem is that any water in system condenses out as separate drop of water, with boiling point of 210 F. Any water drops also can foster corrosion wherever they sit. Finally, Dot 5 silicone fluids are notorious for entraining microbubbles of air that are very difficult to bleed. Rapid cycling of ABS valves will also cause microbubbles to form in silicone fluid, which is why Dot 5 is not recommended for ABS systems.

JrRifleCoach

This post by tkrussell is by far the best short explaination yet.

bogus

Synpower is DOT 4. It's all I use.

TKs post is right on!

Dartvader

Thanks TK. That explanation sort of brings a lot of bits and pieces of information together. Well done. Bogus, do you know if the Synpower can by added to dot 3 as I bleed the brakes? I am doing the c-5 upgrade soon, was going to use dot-3, but if I can simply follow up what is in there with Synpower as I bleed them out, then I will do that.

larryfs

That synpower valvoline failed on me in 8 months. IT absorbed enough water that I boiled the fluid. For street cars I've been using castrol LMA with good results.

bogus

That is the first reported failure of the Synpower that I have seen here. Interesting.... What is your climate like?

PLRX

I use this on all my vehicles.

Davemc1963-87conv

JFYI
DOT 5.1 is compatible with all DOT3 & DOT4 fluids and is suitable for ABS systems. Most of the newer high end fluids are DOT 5.1, like Castrol, Motul, AP Lockheed, etc.
Not to be confused as has already been stated with just 'DOT5' which is Silicon based and must not be used on ABS vehicles.
The really messed up when they came up with the 5.1 designation. If you do a search, you will find several posts on this topic.
Dave

CentralCoaster

In short, Dot 5 will turn into Guiness head when pulsated at higher frequencies, such as by ABS systems. I think it is also more compressive.

Dot 4 (and apparently 5.1) are reverse compatible with Dot 3.

AORoads

Considering the relative cost and amount of "new" brake fluid, why not just bleed out all the old stuff and put in Synpower? Then you know you've got new fluid to go along with new brakes. And you know that if there is moisture in there, mostly, it'll be gone.

Great explanation by you and TK re hygroscopic brake fluids, etc. Thanks!

Tom Piper

tkrussel hit the nail square on the head.

The one thing he didn't elaborate on is there is more than moisture that needs to be addressed by changing the brake fluid.
I'm not sure about DOT5, but the others have corrosion inhibitors that wear out over time.
So, changing the fluid also puts the corrosion inhibitors back....cheap insurance for master cylinders and expensive ABS units.



Tom Piper

runner140*

Between bleeding and flushing, i've used over two quarts of Valvoline synpower. Works for me.

eschoendorff

Same here. It's good and it's relatively cheap and easy to find.