VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Brake Fluid DOT 3 vs DOT 5

I will be rebuilding the wheel cylinders on my 48 soon. Looking for information on DOT 3 vs DOT 5 Brake Fluid:

1) the pros and cons

2) how to of changing from DOT 3 to DOT 5 Brake Fluid.

Any information is appreciated.

DOT 3 is hygroscopic (absorbs moisture). When a brake system is used often, the absorbed water in the brake fluid is "burned off" by useage. In collector cars that sit for relatively long periods of time, the absorbed water can cause rust to form on the inside of brake lines and cotaminate the brake fluid to possible cause failure. DOT 3 fluid is also an excellent paint remover if you happen to spill it.

DOT 5 is silicone based and therefor, not hygroscopic and will not remove paint or cause rust or contamination.

I've used DOT 5 in all of my old cars since 1980 and have never had a problem with it. Some people have reported stoplight switch failures due supposedly, to the use of DOT 5. I have not experienced that on any of my cars. I've also heard complaints of "spongy" pedals with DOT 5, but again, that's never been my experience.

To convert to DOT 5, ALL rubber components (hoses, wheel cylinder & master cylinder rubber) should be replaced and all metal lines filled with solvent and blown clean to remove all traces of DOT 3 - they are NOT compatible.

I just converted another car to DOT 5 and incororated a complete brake job and conversion to a dual master cylinder. IMO, the only way to go.

I'm sure you'll hear descenting opinions....................

I have Dot 5 in my Corvair. Its been in there for about 20 years, back when I went through the system. I have never done any more with it but would flush and refill with fresh if I had the time.
I have had Dot 5 in the '39 for about 25 years. It has been replaced from time to time when I rebuilt wheel cylinders due to rusted pistons. The Bendix brakes have the rubber boots on the ends of the cylinders that keep out the moist air and seldom rust. The adjusting covers on the Huck brakes do not do this. Read EASYMONEY'S post on brakes to see how I now keep the wheel cyl. pistons from rusting.
I replaced the stop light switch on the '39 a few times. It would get so you had to stand on the pedal real hard to get the stop lights to work. I installed a mechcanical switch like the 1942 and up used to solve that problem.
I do like the feature that it will not remove paint.
My other two cars have Dot 3. I bleed them out every few years for sure.

I ran across this interesting article on brakes:

http://www.hotrodheaven.com/tech/brakes/

The last paragraph is on brake fluid and has some intersting thoughts on DOT5. I have DOT5 on my 64, for the benefits listed above, but it looks like there is a downside, in that any moisture that does get in, will collect together and possibly flash to steam if heated enough.

David,
Some of the Quote in the box is correct and some is not.

First: DOT (Department of Transportation) 3 is NOT ethylene glycol based. The base is a mixture of glycol ethers. They are chemically similar to linking ethylene glycol with another glycol (propylene or butylene or ?). They have higher boiling points and lower the water absorption that ethylene glycol. The essentially have zero effect on the swelling of brake system cups.

Second: DOT 4 brake fluid has a different but related base as DOT 3. It has similar properties but higher "wet" boiling point. The "wet" boiling point is the virgin fluid with a specified amount of water added. It was developed for factory fill when disc brakes replaced drum brakes in the early 1970s. Yes I know that Corvette and others had disc brakes before the '70s. It took a few years to realize that a higher boiling fluid was needed for the disc brakes due to higher brake component temperatures.

Third: Silicone brake fluid (DOT 5), developed by Dow Corning (with a bunch of help from Dow Chemical) has a higher boiling point than DOT 4 and lower water absorption. Increased use of disc brakes was a driving factor in its development.

Fourth: It is a little known fact that water and air are drawn through the rubber hoses as well as the master and wheel cylinders or calipers. Each type of fluid has additives to minimize the effect of water, reduce corrosion, increase lubricity, etc. As stated in the quote water disperses into the DOT 3 & 4 fluids but remains as particles or droplets in DOT 5. If it migrates to the calipers it can produce steam which is very compressible with DOT 5. Or in cold weather can freeze to ice in brake lines. Both of those are bad. The problem is worse in systems that are infrequently used as there is no heat and circulation from braking to force the water out of the system or move it to other parts of the brake system where it is less detrimental.

Though I have silicone brake fluids in my inventory, I have never converted the brake systems in any of my cars. When I remember I try to change the DOT 3 or 4 every two to three years at the same time I change the antifreeze. If I let the vehicle remain stationary for many years I often have some brake problems when I get it back out. Does not happen with the old mechanical brake systems. They don't go bad in storage.

This is true if you do not pressure wash the under carriage of your mechanical brake equipt car, then fail to lubricate it before letting it set all winter. I assure you that it takes some heavy lubing and working of the brakes to free them up.

Not that I have done this and know 1st hand. It is just a hypothetical situation that I happen to think up!

Sounds like none of them are perfect. I will just take the non-paint stripping attribute of DOT 5 as a big plus for a car with a detailed engine bay, and call the rest a wash.