[Pecosbill]

Completely new brake system, stainless steel lines, all new wheel cylinders, new master cylinder, etc. About ready to fill 'er up with fluid and have been wondering about which fluid to use. I've read that Dot 3 is pretty much what came with our cars, but that Dot 5 is supposed to inhibit corrosion, especially in a car that sits for a while between drives. But I've also read that some folks had trouble bleeding their brakes with Dot 5. I'd really like to hear what other board members have to say about their choice of fluids.

[Option 382]

You will not have any problems with the DOT 5 that your describing.

[ron cozzo]

I used the DOT 5 that Advance Auto has on the shelf. No issues bleeding and no worries about paint damage.

[geeteeohguy]

Dot 5 does not absorb water like DOT 3 and 4. That said, it is not used in vehicles with ABS brake systems because of this. Moisture does get in a system from heat/cold condensation, etc. and with DOT 5, it stays in one area due to its different density, like gas and oil. This can cause in the modulator valve in an ABS car. The main advantage of DOT 5 is a higher boiling point. The boiling point of DOT 3 is above 446 F. For a car that is not on the road race circuit, I can't personally see using DOT 5. You'd be better off using DOT 3 and changing the fluid every 2-4 years.

[Peter Serio]

Speaking of changing the fluid. I agree. One question what would be the best way to do this? Just bleed the brakes as you would normally and keep introducing new (fresh) clean fluid into the MC until at all 4 bleeder screws, it comes out clean?

[geeteeohguy]

Yes, Pete, and you can install plastic or neoprene hoses to the nipples to keep the fluid off of the suspension/backing plates. What I do, is take a syringe and suck about 95% of the old fluid out of the master cylinder and re-install new fluid. Then, it only takes a little time to clear out the old stuff from the lines. Just leave enough in the master to cover the ports so you don't get air in the master. If you get fluid on any components, it's water soluable, so flush with a water hose and you're good to go.

[Peter Serio]

Hi, ok thanks. I just bought a newer GM car (my everyday) driver. Had it shipped up to me from down south. I noticed the brake fluid in the master (probably the original factory fill) was a bit brown in color. Thought it would be a good idea to refresh that. I just finished up all the brake lines on my '63 Lemans so have been thru the process.

[Terry M. Hunt]

Over the last 35 years I have converted all my cars(6) to DOT 5.

[Pecosbill]

I see there are varying opinions. The reason I was considering Dot 5 is the anti corrosive nature of it and the fact that it is easier on your paint if you happen to spill it. The only real negative I heard was Jeff's comment about not using it in ABS brake systems, which I don't have. I had brake fluid in my first 64 convertible top cylinders way back when and the lines leaked and it ate all of the paint off of the back section of the rocker panels. I guess that's where it found its way out.

[geeteeohguy]

Roger that. I had brake fluid in my '67 GTO's top cylinders and pump when I got the car in the early '80's, and had to remove and overhaul the top motor every year because the impeller would sieze up due to moisture being absorbed by the brake fluid. Dumped the brake fluid and replaced it with ATF 17 years ago, and haven't touched that top motor since. Works perfectly.

[johnsma22]

FWIW, DOT3 and DOT4 brake fluid absorbs moisture by design. No matter how tight you think your brake system is, moisture will and does get in. With glycol based brake fluid, which is hygroscopic, the moisture is absorbed into the entire volume of fluid. Over time this will mean replacing the brake fluid. Silicone brake fluid does not absorb moisture. While this seems to be an advantage, it isn't. Because the moisture is not absorbed into the fluid it ends up in the lowest point of the system, the calipers and wheel cylinders. As you know, water boils at 212˚F, so therein lies one of the problems with DOT5 brake fluid.

Another problem with silicone brake fluid is the fluid itself is compressible. That leads to a spongy pedal feel. I've never felt that the fact that silicone brake fluid doesn't damage paint to really be that big a deal.

If you switch over to the DOT-5 fluid you WILL have to change everything made of rubber in your brake system and flush out your hard line thoroughly, as it is not compatible with DOT-3 or DOT-4. If you do that and decide to switch back, you will have to change out everything rubber again.

The only entity that has gone completely with DOT-5 brake fluid is the US military, due to its superior cold weather properties, but they are in the process of phasing it out.

Here's an article from Super Chevy that sheds some light on the subject.

Moisture, at a molecular level, enters through microscopic pores in the rubber brake hoses, caliper piston and wheel cylinder seals, cracks in the hoses, or when the brake reservoir is opened. This effect is heightened by the hygroscopic nature of the brake fluid itself and the cooling of the brake system from the tremendous heat generated during braking. As little as 3% moisture contamination can reduce the boiling point of brake fluid by 25%.

DOT 5 fluids, not being water miscible, the water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense. This is not an issue in a racing scenario because of the attention that is paid to a racing vehicle, like bleeding the brakes frequently to remove the accumulated water.

The following is taken directly from a technical cut sheet provided to me by Wilwood. Considering that they are one of the leading manufacturers of aftermarket braking components it is safe to assume that they are not making this stuff up just to sell their own brand of brake fluid. I'm sure that their own engineers have thoroughly researched this before making the recommendation against the use of DOT-5 brake fluid in their braking systems.

We do not recommend using DOT 5 fluid in any racing applications. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers . It is not uncommon to have caliper temperatures exceed 200 F, and at 212 F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel.

[geeteeohguy]

Very, very well put, johnsma22. You said what I was trying to say---that the moisture gathers in concentrated areas in the low points of the system,mainly brake calipers and wheel cylinders. On ABS cars, the modulator valve assemblies ($$$$$). Much better off for the pocketbook, performance of the vehicle, and peace of mind to run DOT 3 or 4 fluid and change it out every few years. We have a brake fluid tester at work that does a boiling point test of the fluid to determine moisture content. Where I am, in a semi-arid climate, the fluid stays serviceable for a long, long time. Silicone brake fluid is a bad idea, in my professional opinion as an automotive technician.

[Pecosbill]

What's the difference between Dot 3 and Dot 4?

[johnsma22]

Originally Posted by Pecosbill

What's the difference between Dot 3 and Dot 4?

Basically, DOT 3 meets the minimum specifications required by federal regs for fluid viscosity, temperature and chemical stability, corrosiveness, water tolerance, etc., while DOT 4 meets more stringent requirements. Both DOT 3 and DOT 4 brake fluids are a mixture of polyalkylene glycol ether and other glycols, but DOT 4 fluids add borate esters to raise the boiling point.