Brake Fluid Q

[nvrenuf]

So what happens when you mix DOT4 and DOT5 fluids?

The Air Force crew cab thread indicated that the military uses DOT5 fluid in at least some of their vehicles. I have an Air Force crew cab with a chronically spongy brake pedal since I added longer braided hoses front and rear. Until now I just figured my brake bleeding skills sucked as I've attempted to bleed the system many times using gravity, pressure and vaccum methods (always with DOT4).

Could this be my problem?

 

[Boondocks]

Since neither of the fluids will compress, I doubt it is that.

I would bet that if you new longer hoses are not steel-braided, that they are the cause of the sponginess.

 

[nvrenuf]

The hoses are the steel braided type (Skyjacker).

I've always heard you weren't supposed to mixed the two types of fluid so this made me wonder why and what might happen if you did.

 

[ntsqd]

No idea what happens if you mix DOT 4 & DOT 5, but DOT 5 is SLIGHTLY more compressible than DOT 4 It is a falicy that fluids are incompressible, just they aren't considered compressible at 'normal' pressures. The amount that they compress, as compared to a gas, is extremely slight. However, in the pedal you can feel the difference in compressibility btwn DOT 4 and DOT 5 fluids.

I suggest that you do a complete fluid flush and see what that gains you.

 

[Ruthven13]

DOT 5 fluid does in fact compress slightly. DOT 5 is silicone based and will NOT mix with other brake fluids. They seperate like oil and water. You have to completely bleed the whole system to get all of the 5 out. The good thing about it is that is doesn't absorb water and I think it has a relatively high boiling point.

 

[nvrenuf]

Good idea, I will try that this weekend. Any reason I shouldn't do the flush with (and continue to use) DOT4?

 

[Boondocks]

Hmmm. Learn something new everyday.

I knew that fluids will compress a minimal amount. But, never thought they would compress a noticeable amount. I certainly never thought the difference in compressibiliy between two different brake fluids would be noticeable.

Thanks for that.