Quote Originally Posted by 64 Traveler View Post
When I put the big brakes (11"), new rubber lines and rebuilt the master cylinder on my Willys I went with DOT 5 brake fluid. When I rebuilt my master cylinder it had rust in it, as you know rust comes from water. I went with DOT 5 because my master clyinder is under my Willys where all the water is. I've been using DOT 5 for at least 16 years with no problems other than needing a brake light switch every 6-7 years.

I disagree with your statement about DOT 3 not absorbing water, there are good reasons to use DOT 3 but it's a magnet for water. If you still have the original master cylinder under the floor in your Willys, it will get water in it. From the article you linked...

"Non-silicone-based brake fluid (DOT 3) is strongly hygroscopic, meaning that it naturally absorbs water from the humidity in the air. That is why the shipping container and the brake reservoir have to function as a barrier to the moisture in the air reaching the brake fluid. Modern brake reservoirs are thick enough, and the bellow seals on top function well enough, to provide a long life to the fluid once in use."

Modern brake cylinders is NOT us if you have the stock stuff under your Willys.

From the article...

DOT 3 fluids are usually glycol ether based, but as stated earlier, that is not because they are required to be. The brake fluid industry has determined by consensus that glycol ether fluids are the most economical way to meet the requirements.

DOT 4 fluids are also glycol ether based, but have a measure of borate esters added for improved properties including increased dry and wet boiling points. A seldom talked about characteristic is that because of this chemistry, the DOT 4 fluid will have a more stable and higher boiling point during the early portion of its life, but ironically once the fluid does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT 3.
The real differentiating factor is that DOT 4 fluid should be changed more often than a DOT 3 fluid, because of the effects and rates of water absorption.

The original DOT 5 fluid specification was expected to be fulfilled by silicone based (SSBF) composition. It was designed for use in applications where its resistance to water absorption (and therefore low corrosion) was desired - like in military equipment. It has also found use in antique cars because it does not dissolve paint finishes.

As I said, brake fluids involve compromises. Further reading on the topic: