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Dave's Garage

Dave Sander [ July 2014 ]

Brake Fluid

This month we have a question about
brake fluid from Ed Hilbert.

Hello David,

I am redoing all of my Mercedes brake components except for the steel brake lines which appear to be in good shape. What are the different types of brake fluid and when to use each type? Would I do well to change over to the newer silicone brake fluid or stick with the standard DOT type 3?
What are the advantages of the silicone over DOT 3 perhaps silicone wont absorb moisture and thus lessens the chance of rust? If I do change over, must I completely flush out the old fluid and if so with what alcohol? How incompatible are the two types of brake fluid?

What is brake fluid made of? Why is it used instead of standard motor oil?

Can it be used as a paint stripper? If so, how would one clean it off the surface so paint would stick again?

Anything else we should know about brake fluid?

Thanks for your expertise!
Ed

Dear Ed,

The types of brake fluid are DOT 3, DOT 4, DOT 5 and DOT 5.1. DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). Polyethylene glycol fluids are hygroscopic and will absorb water from the atmosphere, necessitating a flush/replace every couple of years. Polyethylene glycol fluid WILL absorb moisture. Failure to replace contaminated brake fluid will lower the boiling point of the brake fluid, and the moisture will cause rust and corrosion of the brake system. Silicone, DOT 5 fluid will absorb just a minuscule amount of water.
Brake fluid is classified by its boiling point. The "dry" boiling point is with no moisture in the fluid. The "wet" boiling point is brake fluid with moisture in it. "Wet" brake fluid is defined as having 3.7% water by volume.

Dry boiling point Wet boiling point

DOT 3 205 C (401 F) 140 C (284 F)

DOT 4 230 C (446 F) 155 C (311 F)

DOT 5 260 C (500 F) 180 C (356 F)

DOT 5.1 260 C (500 F) 180 C (356 F)

I would highly recommend changing over to the DOT 5 Silicone fluid. Cars that I switched over to Silicone in the early 1980s have no issues, and the fluid continues to function well.

DOT 4 brake fluid has a higher boiling point than DOT 3, and is specified by many manufacturers for better braking performance.DOT 3, 4 and 5.1 brake fluid will remove paint, but there are much better paint removers. DOT 5 fluid, Silicone, will not harm paint. If it is accidentally spilled on the paintwork, it will not damage the paint, and can easily be wiped off.

I would imagine surfaces contaminated with brake fluid could be cleaned up successfully before painting.

DOT 5 Silicone brake fluid does have some drawbacks. It is very expensive, much more expensive than DOT 3 and 4. DOT 5 fluid can absorb air bubbles, and these air bubbles take some time to settle out. It can be difficult to bleed the air out when bleeding the brakes.
Silicone fluid can not be used on vehicles with anti lock brakes.
I have been wrenching for years, and I have fixed countless cars with damage caused by moisture in contaminated brake fluid. I have had to free frozen pistons, and replace wheel cylinders, calipers and master cylinders due to rust and corrosion. I have also had my brakes fail due to the boiling point of the brake fluid being so low, that the fluid boiled resulting in the total failure of the brake system. Old brake fluid is extremely dangerous, as it can and will cause the brakes to fail without warning.

The ideal time to switch to Silicone fluid is when the rubber parts are all being replaced. Blow the metal lines out with compressed air, assemble the brake system and then flush with Silicone fluid until clean fluid comes out of the bleeder screws.

While somewhat expensive up front, if your car is a long term investment Silicone brake fluid will pay for itself many times over, saving you the cost of perpetual fluid replacement, the cost of repairs due to corrosion of brake components, and the cost of paint repair due to spilled brake fluid.


Please email all inquiries to: Dave
or snail mail
32 Turkey Hill Road
Richmond VT 05477

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