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Ok Boys & Girls:
What is the difference between DOT 3,4,5 and why should I use one over the other ?
Going into a 54 1/2 ton with new brake cylinders and lines. Some fluid still in Master. Master has been rebuilt or New
( Gloss black paint still clean...rest of undercarraige & stuff looks like a 54 ).
Thanks

DOT 3 and 4 are glycol based, chemically similar to antifreeze. The only difference is the wet boiling point, 4 is higher than 3. DOT 3 is the same as the Delco Super 11 that was the oem fill. DOT 4 is also ok.
DOT 5 is a silicone based fluid, used mainly in motorcycles. It has a higher boiling point that the glycol based 3 and 4. It also does not absorb moisture like the glycol fluids. There are potential compatability concerns when using in inautomotive brake systems that did not use it as oem. As long as all of the rubber components are certified for use with silicone fluid, it should be ok.
There are a couple of warnings if using the DOT 5, one concerning the mentioned compatability. If the rubber is not DOT 5 compatible, it can cause premature seal failures. Some cases, the seals do not swell enoung, others it swells them too much and makes them go mushy.
The other issue, which is talked about in Bob Adlers column in the current issue of Vintage Truck, is the non-hygroscopic nature of the silicone fluid. If moisture does get in the system, instead of blending in the fluid, it makes a "slug" of water, usually at the low point in the line. This can cause a mushy pedal, especially whan the brakes get hot. Another problem, which Bob talks about, is the potential for the water to freeze in the lines, making the brakes non-functional.
There is one other fluid on the market, DOT 5.1, which is a glycol based fluid with a very high boiling point.
My personal opinion, based on both my experiences with using DOT 5 and my job experience as a brake mechanic, stick with glycol.

Longbox55...Thanks for the very good explanation of the different types of brake fluid. A couple more question if you don't mind. Some of us don't put many miles on our old rides so they sit for long periods. Since DOT 5 doesn't absorb water, would it be a better choice than others? How the heck do you tell what will or will not work with DOT 5? When building up a brake system we just ask for disc/drum and piston size. Lastly, what is necessary to go from one type to another after the system has been "wetted"? Thanks and please forgive for kind of stealing this thread.

I personally do not feel it is best, but I had a bad experience with DOT 5 causing leaks due to incompatability. You would have to contact the parts manufacturer to confirm compatability. To my knowledge, EIS was the only company that certified its parts to be 100% compatible with DOT 5, but they are no longer in business.
DOT 5 is also known for being hard to bleed. Thsi is partly due to the nature of the fluid. It has a tendacy to get "mivro bubbles" in it, especially when first poured. It's best to let it sit to allow the bubbles to naturally settle out of the fluid. No harsh pumping of the pedal (I know lots of guys guilty of that), easy strokes, otherwise it will cavitiate. Of done properly, it will have the same pedal feel as glycol based fluid.
As far as fluid type, it is best to use only the fluid that will be run in the system, as the two types are not compatible to be mixed. If you were to do a change, the sytem should be flushed thoroughly with denatured alcohol, and preferrably all rubber replaced. This is partly to prevent contamination, but also there is a potential for swelling/softening of the rubber, which can lead to brake failure.
I won't say not to use DOT 5, there's several on here that have used it successfully. I'll only say to do the research, make sure the parts will work with it, and follow the bleeding directions to the letter for DOT 5.

I use DOT3 and change/flush it out every few years.

cm

The only time I used DOT 5 it was for my SCCA race car. And that was because the car was constantly being torn down for service and brake work.

For anything on the street, I use Ford's HD DOT 3.

This is awesome I was going to put the DOT 5 in my new brake system. I'll stick with the DOT 3 - with my little experience I'll have all I can hndle with the DOT 3, never mind the DOT 5.