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In my ‘37 1/2 ton: Considering everything is new; master cylinder, slaves and lines, what are opinions of using DOT 5 silicone? Pros and cons

This has been discussed over the years. e.g.:
Brake Fluid Discussion

I chose DOT5. It's nice not to worry about spilling paint remover on the floor when topping up the master cylinder.

thanks Gord, that link was pretty informative. I wonder if opinions have changed much since those articles and threads are pretty old.

I've had Dot 5 in my 1939 for more years than I can remember. Never had a problem.

Rick

early posts had concerns with what the silicone did to the rubber, most others pooh pooh that, that the DOT rating says it should be safe for the seals. Pooling of moisture and the bake light switch seem to be legitimate concerns. Any experiences there? Maybe like all things mechanical, a bit of luck doesn't hurt.

My only experience with DOT 5 was helping a neighbor restore a mid-70s FMC (Food Machinery Corp.) RV, that a previous owner had converted to DOT 5. This vehicle was from Phoenix, AZ and all of the cylinders were ruined by rust, apparently resulted from water layering out as I recall beneath the silicone. How that vehicle got that much moisture in the cylinders I don't know, maybe it was stored in a lot that flooded during our summer rain? He had to replace the whole system.

From my perspective, DOT 3 or 4 is hygroscopic which means it absorbs water. This is actually good, at least to a point, so that a little water does not affect performance or cause pooling and any water can be removed by periodic bleeding. Periodic bleeding is really very simple and inexpensive, so routine maintenance will remove any moisture that gets in and you won't have rubber problems like you might with DOT 5. My 2016 Silverado uses DOT 3 so the engineers must think it is still a good choice (or was GM just trying to save a dollar or two on each truck?).

Clearly, many people have had success with DOT 5 and I would not try to convince them otherwise, especially if it was installed in a new system. A skeptic would rightfully claim my apparent fondness for DOT 3 might stem from the fact I live in a very dry area. although I would point out that my '92 Jeep Wrangler crosses creeks and washes and the brake system has been maintained in fine shape by routine bleeding of it's DOT 3 system.

Kent

I use DOT 4. Two major differences from DOT 3. 1) Higher dry and wet boiling points. 2) Most DOT 4 fluids contain a borate ester. The borate ester reacts out water so it is not corrosive until enough water has entered to use up the borate ester.. Look for phrases like "low moisture activity", "low water sensitivity", etc. Some product data sheets (not MSDS) list a borate ester ingredient.

thanks Harold, I was wondering if anyone would weigh in on DOT 4 or 5.1. It seems the boiling point wet and dry points to DOT 5.1 being the best in that aspect. How is 5.1 for water absorbtion compared with dot 4? Why it not a more common upgrade?

Back when I was working DOT 5.1 was originally developed as a glycol ether based alternative to silicone based fluids. I had very little personal experience with it. The higher wet boiling point could indicate borate ester technology or the differences in ingredients and formulation. I haven't done a lot of searching for product data sheets. I doubt that many are easily available that have that information.