1995 Hardbody: Brake fluid question

[james11]

I thought I read in the Service manual that DOT4 brake fluid had a different compressibility than DOT 3 and it would mess up the ABS. But some DOT 4 and 5.1 fluids say they're compatible. Is it okay to flush out the brake and clutch lines, and replace the Dot 3 with a compatible Dot 4 or 5.1 such as ATE 200 or Valvoline Dot 3/4?Also how often should the clutch and brakes be flushed?

[Desert Rat]

I'd go back to DOT3. With regard to flushing, how does the fluid look? Is it brown and burned looking? If so, it wouldn't hurt. if it's still relatively clear, you're probably fine. Same with Clutch fluid. The only way those fluids really go bad is from burning/overheating the fluid, or getting moisture trapped in the system.

[C-Kwik]

My understanding is the only chemical difference between DOT 3 and DOT 4/5.1 is that DOT 3 is made of glycol and glycol esters. DOT 4/5.1 use the same chemicals, but add borate esters to increase the boiling point. Both mix easily as both are primarily glycol based, so any compatibility issues would likely be mostly the result of seals that are not compatible with the borate esters (Though, this is my speculation based on known differences; I have not found anything that states this explicitly).

As for compressibility, I've run DOT 3 and DOT 4 fluids and I've never noticed any differences in its compressibility. I have also not been able to find any technical info that states that DOT 4 is more compressible than DOT 3.

DOT 5 fluid is silicone based and actually is much more compressible than glycol based fluids. Perhaps that is what you read. ABS generally has a hard time with silicone based fluids as they can form air bubbles when cycled rapidly.

[james11]

As for compressibility, I've run DOT 3 and DOT 4 fluids and I've never noticed any differences in its compressibility. I have also not been able to find any technical info that states that DOT 4 is more compressible than DOT 3.

I may have confused compressibility with viscosity. I think Dot 3 is a little thinner than Dot 4, and Dot 5.1 is thinner than both of those. I think maybe brake fluid that's thicker than Dot 3 messes up the ABS, but Dot 5.1 manufacturers are advertising their thinner fluid as working well with ABS.This link refers to why Dot 4 isn't recommended by Nissan: http://en.allexperts.com/q/Nis...2.htm

Modified by james11 at 11:38 PM 12/22/2008
Modified by james11 at 11:39 PM 12/22/2008