Brakes: Brake fluid?

[Surfpilot]

Im in the middle of rebuilding the split master cylinder on the 63 Lark, and as I'm grinding the rust off the cover, I see it says to only fill with SAE 70-r3. Somewhere in the recesses of time I think I've heard of this fluid, but have certainly never seen it on the shelf. I think we used DOT 3 last time the brake fluid was addressed...however I seem to remember that DOT 3 will destroy old rubber parts...Which might be why I'm rebuilding it now. Anyone come up against this issue? Is there a SAE 70r3 equivalent? Is DOT 3 ok?

[jackb]

Dot 3 is OK...

[StudeRich]

It doesn't actually destroy Rubber Parts but may damage them from wearing on the rusty pitted surfaces it creates on all 5 Cylinder walls due to the alcohol in the fluid attracting moisture and absorbing it.

That would be why DOT 5 Silicone is better, but requires all Brake components be totally clean, flushed and best replaced before installation of DOT 5.

[Surfpilot]

Thanks guys

[RadioRoy]

DOT 4 is compatible with DOT 3 and is not hygroscopic like DOT 3 is. That means that DOT 4 will not attract moisture from the air like DOT 3 will.

[sweetolbob]

DOT 4 is compatible with DOT 3 and is not hygroscopic like DOT 3 is. That means that DOT 4 will not attract moisture from the air like DOT 3 will.

Just to be on the safe side. Both Dot 3 & 4 are polyglycol based and that's why they are compatible. The operative term is Dot 4 adsorbs water at a SLOWER rate and has better high temperature performance. It's a better choice as Roy says but it does absorb water.

Bob

[RadioRoy]

Just to be on the safe side. Both Dot 3 & 4 are polyglycol based and that's why they are compatible. The operative term is Dot 4 adsorbs water at a SLOWER rate and has better high temperature performance. It's a better choice as Roy says but it does absorb water.

Bob

Thanks for the clarification.

If I had written my explanation more clearly, I could have used the phrase "as much as" instead of the work "like."

[bosshoss61]

good thread----but how does the moisture get in there to begin with when or if the system has no leaks and is full of fluid?

[bezhawk]

Dot 3 fluid is hygroscopic.....it absorbs water out of the humidity in the air.

[sweetolbob]

Dot 3 fluid is hygroscopic.....it absorbs water out of the humidity in the air.

As is Dot 4, just at a slower rate.

Bob

[jnormanh]

good thread----but how does the moisture get in there to begin with when or if the system has no leaks and is full of fluid?

The master cyl is vented, and a little air and moisture get in. Cars which are driven regularly get the brake fluid hot, and some of the water evaporates. Cars which are seldom driven should have the fluid flushed once every year or two.

[Skip Lackie]

The master cyl is vented, and a little air and moisture get in. Cars which are driven regularly get the brake fluid hot, and some of the water evaporates. Cars which are seldom driven should have the fluid flushed once every year or two.

Right. Every time you step on the brake pedal, some brake fluid is forced out of the master cylinder into the brake lines, and at least a little air is sucked into the m/c to take its place. And seasonal and diurnal temperature changes do the same thing, albeit more slowly. Modern m/cs have an accordian-like expandable gasket that fills in some of the space, but there's always a little exchange of air in and out of the m/c.