Break Fluids

[Ray C.]

I have read many post in the archieves on break fluids and there seems to be a difference of opinions on which type to use. I have a completly restored break system in my 1965 restoration project and it is time to install the fluid and bleed the breaks. My question is: What is the best type of fluid to use in a restored system?

Thanks! Ray

[Duke W.]

Re: Brake Fluids

Assuming your master cylinder and calipers were put together with conventional glycol-based fluid you must use glycol fluid. I use DOT 4 glycol brake fluids, which have higher boiling points than DOT 3 blends. Glycol-based fluid should be replaced every two to four years, depending on your climate. Damp climates - every two years - up to four if you live in the desert. If you don't replace the fluid the absorbed moisture will cause internal corrosion. Back in the seventies it cost more to rebuild Corvette disk brakes than the cost to rebuild a small block engine. It's relatively cheaper nowadays, but not trivial.

DOT 5 silicone brake fluid does not absorb moisture, so it doesn't need to be changed as often, but if you want to use silicone, you'll have to disassemble everything, clean everything thoroughly with alcohol to remove all traces of glycol fluid, and reassemble with silicone fluid.

I only recommend a swap to silicone fluid if you start from the ground up with EVERYTHING clean and dry. Mixing glycol and silicone fluids is a recipe for trouble.

Duke

[Jim T.]

Re: Brake Fluids

Duke you mention that silicone fluid does not need changing as often as DOT3/4. What is a recommended time change for silicone?

[John D.]

Re: Brake Fluids

Duke:

I think the previous owner of my car mixed the fluids. I have rebuilt the MC, all calipers and replaced the 4 hoses and trailing arm lines. Using a compressor I blew the lines out, ran denatured alcohol through the lines and blew out with air again. In your opinion are they clean enough? Or should I run alcohol through them again? I am planning on using silicone.

John

[Duke W.]

Re: Brake Fluids

Jim - I don't have a change interval range for silicone. One thing that occurs in the brake system is particle contamination. Bits of mold release and rubber break loose over time. This can be detected by a combination of color change and debris at the bottom of the m/c. Flushing the system should remove some of these particles, so I would flush it when you notice a signficant color change
or detect debris in the m/c.

If neither of these occur, I'd probably flush it at least every 10 years.

Duke

[Clem Z.]

Re: Brake Fluids

blowing the lines out with compressed air is not a good idea because of the water in the compressed air. you should use dry nitrogen to blow out the lines. also never leave the system opem to air even when thhere is no fluid in the system.

[Duke W.]

Re: Brake Fluids

When you assembled the m/c and calipers, what did you use as an assembly fluid? If you plan to use silicone fluid, everything should be assembled with silicone fluid. If you used glycol fluid, take everything apart, clean everything with denatured alcolhol to remove all the glycol fluid and reassemble everything with silicone fluid. I'd also recommend flushing the lines once more since some glycol fluid may have seeped in.

The key to successful use of silicone fluid is to start with everything clean and dry, then assemble the m/c and calipers/wheel cylinders with silicone fluid. If the brakes pipes are still on the car they can be flushed with alcohol and thoroughly dried with compressed air. I'd even recommend flushing out new rubber brake hoses.

Duke

[Craig S.]

Re: Brake Fluids

Does anyone know what Vette Brakes uses to assemble and test their calipers? I don't really want to disassemble a brand new set of calipers for my 67 that were put together for NCRS judging...thx, Craig

[bruce11495]

Re: Brake Fluids....Silicon NOT

Are you going to drive this car on a regular basis? If so, don't even think of using silicon fluid...There are so many disadvantages to using it that even Dow Corning, one of the main producers, does not, NOT reccomend it for street use.
Use a premium quality fluid, Castrol GT LMA (low moisture absorbancy), and change it every 2 years............

[Terry M.]

Re: Brake Fluids....Silicon NOT *TL*

Bruce,
Please supply me with the DuPont quote against street use of DOT 5 silicone brake fluid. I have a potential story on the subject and I have been accumulating references before I decide to print the story.
Terry

[Duke W.]

Re: Brake Fluids

Suggest you call and get the straight story from them rather than relying on someone else's opinion or guess.

Duke

[Duke W.]

Re: Brake Fluids....Silicon NOT

Yeah,I'd be interested, too. Many report "problems" with silicone fluid, but I believe they are caused by misapplication rather than inherent problems with the fluid itself.

Many try to change to silicone using the "bleed to flush" method and this will usually lead to nothing but trouble. Silicone and glycol fluid are not miscible, so there is NO WAY you can completely purge all the glycol fluid by bleeding. Silicone fluid has a signficantly higher specific gravity, so glycol fluid will migrate to high spots and get trapped, and the absorbed moisture will corrode.

Silicone fluid also has a reputation for trapping or dissolving air, which I've never been able to verify with lab testing - shook up a beaker of it once, let it sit for a couple of minutes, then pulled a vacuum on it and I couldn't see any air bubbles.

Silicon fluid is also more viscous than glycol, so it might be a bit slower to respond, but this might only be an issue on drum brake systems where you have signficant fluid displacement when you depress the pedal. With constant contact four wheel disk brakes there is very little fluid displacement, so viscosity effects should be negligible.

The key to successful silicone fluid conversion is starting with everything clean and dry - as new. Then assemble everything with silicone fluid. If you buy commercially rebuilt m/c and calipers/master cylinders that were assembled with glycol fluid, either use glycol fluid or disassemble and clean everything with alcohol and reassemble with silicone fluid.

Duke

[Clem Z.]

Re: Brake Fluids....Silicon NOT

duke was not this silicone brake fluid business started because the early disk brake corvettes had bad weather seal and water got into the system thru these seals. i have now a 15 year old silverado with the original brake fluid and never had a brake problem with any older car with the orignal fluid.

[Jim T.]

Re: Brake Fluids....Silicon NOT

Terry lets do a test if you are using DOT3. I have been using silicone for about 15 years in my orignal owner 70, we could meet at Bowling Green and do a stopping test to compare silcone fluid and DOT3 in our cars. I think by brakes are really good with silicone.

[Duke W.]

Re: Brake Fluids....Silicon NOT

I recall hearing about silicone fluid when I was SCCA racing in the mid-seventies. A lot of racers used it because of it's higher boiling point. That was also about the time I heard about the moisture absorbtion properties of glycol fluid and also when I started hearing about the problems with Corvette disk brakes and the $1200!!! rebuild cost.

There seems to be a lot of variation in the rate that failures occur in glycol fluid brake systems, and I think it has a lot to do with your driving conditions and climate in additon to specific design features of each individual system. The master cylinder failed in my SWC in '68 when the car was only five years old, but it was a daily driver in Seattle. If I lived in Phoenix and only drove the car in mild weather, the original cylinder might still be okay, even without fluid changes. The early Corvette disk brake systems tended to pull in moisture and other contaminants from the piston seals, and it wasn't until the mid seventies that Chevrolet figured this out and modified the design. McClellan discusses this in his book.

The two year change interval for glycol based brake fluid is commonly used by the German OEMs, but the domestic OEMs don't recommend it in their maintenance schedules. Also, most German OEM systems have a vent hole in the m/c cap, but domestic systems have a "semi-sealed" system with the rubber diaphram, so maybe they think this design absorbs less moisture. In any event, the fluid does break down with heat and become contaminated with rubber and mold release particles, so flushing periodically to get all this junk out is a good idea. I can detect a darkening of the fluid in the Cosworth Vega after a track weekend, and it's primarily in the front reservoir, which supplies the front brakes where all the heat is generated because my bias is set up about 80/20.

In 1981 when I replaced the front brakes on the CV with the vented rotors and larger calipers from the Monza I disassembled the rear wheel cylinders and found corrosion in the bores. That was enough to make me a believer, so now all my cars get a bienniel brake fluid change, and I have not had to repair a hydraulic component since then, so it's 22 years and still counting.

Duke