Brake fluid

[John Fulton]

I have recently done some work which required that I disconnect the rear brake calibers (this is a 70). I now need to bleed the brakes, however I dont know what fluid is currently in there. The fluid that came out of the calibers is clear and I know that the calibers were rebuilt 1995. I went through the restorer articles on brake fluids and was surprised that most of the articles did not necessarily recommend dot 5, but seemed to instead favor dot 3 along with yearly fluid changes. I had the impression that hands-down dot 5 was the fluid of choice (except maybe in the mountains). If I have dot 3 now and bleed with 5 or vice versa will I damage my system. Since I do not know what it is should I just bite the bullet and bleed them completely?

Opinions please.

[Joe L.]

Re: Brake fluid

John----

In order to CHANGE fluid TYPE you really need to do a whole lot more than just "flush" the system in a conventional manner. You need to remove all of the calipers, completely disassemble, clean and re-assemble(hopefully with new seals and boots), do likewise for the master cylinder, replace rubber brake lines, and vacuum extract all old fluid out of the brake lines.

In case you'd rather not go through all this and, instead, want to simply identify the fluid you currently have installed and continue to use it, I'd keep the following in mind: generally DOT 3 or 4 glycol-based brake fluid is a light straw color, almost clear. DOT 5 silicone fluid, when fresh in the bottle, is purple in color. The purple coloration is a dye which is added to the fluid to distinguish it. However, once in a brake system the purple dye eventually changes to a yellow color. However, this yellow color is distinctly yellow and not the light straw to clear color of glycol fluid.

In addition, silicone fluid is completely insoluble and immiscible in water. Glycol fluid is completely miscible in water. Consequently, take about an ounce or so of the fluid that is currently in your system and put it in a small, clear glass jar. A tall, thin jar or vial would be preferable. Then add about a tablespoonful of water. Shake the jar/vial and then leave it set for several hours. If the fluid is silicone, you will then note a distinct "phase seperation"(i.e. two different "layers" like oil in water). If the fluid is glycol, you will see a homogeneous mixture with no phase seperation. If you wish , you could establish a "control" by performing this test with a known sample of glycol fluid and a known sample of silicone fluid. That way, you'll know exactly what you're looking for.

[Joe L.]

Re: Brake fluid

John----

In order to CHANGE fluid TYPE you really need to do a whole lot more than just "flush" the system in a conventional manner. You need to remove all of the calipers, completely disassemble, clean and re-assemble(hopefully with new seals and boots), do likewise for the master cylinder, replace rubber brake lines, and vacuum extract all old fluid out of the brake lines.

In case you'd rather not go through all this and, instead, want to simply identify the fluid you currently have installed and continue to use it, I'd keep the following in mind: generally DOT 3 or 4 glycol-based brake fluid is a light straw color, almost clear. DOT 5 silicone fluid, when fresh in the bottle, is purple in color. The purple coloration is a dye which is added to the fluid to distinguish it. However, once in a brake system the purple dye eventually changes to a yellow color. However, this yellow color is distinctly yellow and not the light straw to clear color of glycol fluid.

In addition, silicone fluid is completely insoluble and immiscible in water. Glycol fluid is completely miscible in water. Consequently, take about an ounce or so of the fluid that is currently in your system and put it in a small, clear glass jar. A tall, thin jar or vial would be preferable. Then add about a tablespoonful of water. Shake the jar/vial and then leave it set for several hours. If the fluid is silicone, you will then note a distinct "phase seperation"(i.e. two different "layers" like oil in water). If the fluid is glycol, you will see a homogeneous mixture with no phase seperation. If you wish , you could establish a "control" by performing this test with a known sample of glycol fluid and a known sample of silicone fluid. That way, you'll know exactly what you're looking for.

[Gary Schisler]

Re: Brake fluid

Quite honestly, there is nothing wrong with Dot 3/4 fluid for a Corvette with stainless steel brakes. The only problem with Dot 3/4 is the water affinity. I bleed my brakes every year just to insure that all the accumulated impurities are removed. There is an underlying falsehood out in there that SS brakes are the answer for those who never want to change, clean, or rebuild calipers. The ONLY advantage to SS brakes is that the calipers won't rust. You still need to do periodic maintenance on them, including new seals, new fluid, and the occasional teardown and cleanup. I use Dot 4 and the only difference between that and 3 is the boiling point is a little higher. Since I bleed the brakes every year, I don't worry about Silicone fliud (Dot 5). As Joe said, if you change to Dot 5, you must do a complete teardown, cleaning, and rebuild. Then you must clean out the lines and reservoir with a solvent, run enough Dot 5 through the lines to clear out the solvent, then refill, then bleed the brakes. Sounds like a lot of work to me for very little benefit in my case. I guess that is why they invented chocolate, strawberry, and vanilla. Each to his own.

[Gary Schisler]

Re: Brake fluid

Quite honestly, there is nothing wrong with Dot 3/4 fluid for a Corvette with stainless steel brakes. The only problem with Dot 3/4 is the water affinity. I bleed my brakes every year just to insure that all the accumulated impurities are removed. There is an underlying falsehood out in there that SS brakes are the answer for those who never want to change, clean, or rebuild calipers. The ONLY advantage to SS brakes is that the calipers won't rust. You still need to do periodic maintenance on them, including new seals, new fluid, and the occasional teardown and cleanup. I use Dot 4 and the only difference between that and 3 is the boiling point is a little higher. Since I bleed the brakes every year, I don't worry about Silicone fliud (Dot 5). As Joe said, if you change to Dot 5, you must do a complete teardown, cleaning, and rebuild. Then you must clean out the lines and reservoir with a solvent, run enough Dot 5 through the lines to clear out the solvent, then refill, then bleed the brakes. Sounds like a lot of work to me for very little benefit in my case. I guess that is why they invented chocolate, strawberry, and vanilla. Each to his own.

[Jack H.]

Re: Brake fluid

Issue between DOT 3/4 and DOT 5 revolves around trading the devil you know for the devil you don't know. Set aside the complications of today's anti-lock breaks and the 'traction control' tricks they do by manually pumping the master faster than you can which introduces fluid cavitation issues, and look at a simple classic Corvette with human brake actuation only.

Like Joe says, the DOT 3/4 chemistry is hydroscopic. This means it has an affinity to 'gobble up' water. Water in the brake lines can/will start rust inside as well as at key piston interface points. This is known as 'wear' and one of the reasons it's a real good idea to go into a serious preventative maintenance mode with your older classic Corvette. Also, the reason GM says to change fluid on a periodic basis, that few novice drivers ever do. That's the devil you know....

Now, DOT 5 is a silicon (watch spelling -- with an 'e' on the end you've violated a trademark of Dow Corning, guys) based fluid. It's NOT a hydroscopic fluid, so all the drawbacks go away, right? Wrong, this system's Achilles' Heel is it's areoscopic (likes to gobble up air).

Over time it's possible for the system to slurp tiny air bubbles and bingo, you hit the brakes and they aren't there because natural cavitation struck. It's not a mountain issue. In fact loading/using DOT 5 at high altitude can be a help. It's the guys that fill their systems with DOT 5 at low altitude, don't get/keep the air out, don't inspect/change the fluid because it's 'UNNECESSARY' then go take a spin in the mountains where ambient barometric pressure and O2 density is probably lower, the areoscopic 'load' in the DOT 5 unloads and 'no brakes' on a mountian road when you need 'em REAL BAD!!!

Filling/using at high altitude and driving the car down to sea level, can actually help the issue....

[Jack H.]

Re: Brake fluid

Issue between DOT 3/4 and DOT 5 revolves around trading the devil you know for the devil you don't know. Set aside the complications of today's anti-lock breaks and the 'traction control' tricks they do by manually pumping the master faster than you can which introduces fluid cavitation issues, and look at a simple classic Corvette with human brake actuation only.

Like Joe says, the DOT 3/4 chemistry is hydroscopic. This means it has an affinity to 'gobble up' water. Water in the brake lines can/will start rust inside as well as at key piston interface points. This is known as 'wear' and one of the reasons it's a real good idea to go into a serious preventative maintenance mode with your older classic Corvette. Also, the reason GM says to change fluid on a periodic basis, that few novice drivers ever do. That's the devil you know....

Now, DOT 5 is a silicon (watch spelling -- with an 'e' on the end you've violated a trademark of Dow Corning, guys) based fluid. It's NOT a hydroscopic fluid, so all the drawbacks go away, right? Wrong, this system's Achilles' Heel is it's areoscopic (likes to gobble up air).

Over time it's possible for the system to slurp tiny air bubbles and bingo, you hit the brakes and they aren't there because natural cavitation struck. It's not a mountain issue. In fact loading/using DOT 5 at high altitude can be a help. It's the guys that fill their systems with DOT 5 at low altitude, don't get/keep the air out, don't inspect/change the fluid because it's 'UNNECESSARY' then go take a spin in the mountains where ambient barometric pressure and O2 density is probably lower, the areoscopic 'load' in the DOT 5 unloads and 'no brakes' on a mountian road when you need 'em REAL BAD!!!

Filling/using at high altitude and driving the car down to sea level, can actually help the issue....

[Wayne]

Re: Brake fluid

Let me offer a contrary opinion.... I've used silicon brake fluid in about six different cars, starting in the late 70's. None of them were Corvettes. In each case I've flushed the system generously with dot 5, but didn't disassemble brake cylinders in all cases (only did it if the changeover was part of a brake rebuild). I've not had any problems with dot 5. I just do normal daily driving, no races or autocross, so I can't testify about harsh use. At least one of those cars was driven over the mountains (Lake Tahoe) with no discernible brake problems.

[Wayne]

Re: Brake fluid

Let me offer a contrary opinion.... I've used silicon brake fluid in about six different cars, starting in the late 70's. None of them were Corvettes. In each case I've flushed the system generously with dot 5, but didn't disassemble brake cylinders in all cases (only did it if the changeover was part of a brake rebuild). I've not had any problems with dot 5. I just do normal daily driving, no races or autocross, so I can't testify about harsh use. At least one of those cars was driven over the mountains (Lake Tahoe) with no discernible brake problems.

[Jack H.]

Re: Brake fluid

There's no issue that mountain use is a 'killer' to DOT 5. The issue is how well was the system cleared of air on fill and how tight is the system against the re-entry of air. If all is nice & tight and fill is done properly, DOT 5 works A-OK. Is used under race conditions routinely but these cars see frequent maintenance and are driven by pros who learn to anticipate problems.

Where things break down is with systems in need of rebuild, system fill that's fast and loose, cars that aren't maintained too well and exotics like ABS that weren't designed to use the fluid. It's just a happenstance that high altitude makes problems surface. My 396 trailer queen has DOT 5 in 'er and I'm a half mile above Denver. But, I don't drive it that much and wanted the protection against water. My '71 daily driver has DOT 4 and I change the fluid regularly. It's an owner's call that's not without advocates as well as controversy....

[Jack H.]

Re: Brake fluid

There's no issue that mountain use is a 'killer' to DOT 5. The issue is how well was the system cleared of air on fill and how tight is the system against the re-entry of air. If all is nice & tight and fill is done properly, DOT 5 works A-OK. Is used under race conditions routinely but these cars see frequent maintenance and are driven by pros who learn to anticipate problems.

Where things break down is with systems in need of rebuild, system fill that's fast and loose, cars that aren't maintained too well and exotics like ABS that weren't designed to use the fluid. It's just a happenstance that high altitude makes problems surface. My 396 trailer queen has DOT 5 in 'er and I'm a half mile above Denver. But, I don't drive it that much and wanted the protection against water. My '71 daily driver has DOT 4 and I change the fluid regularly. It's an owner's call that's not without advocates as well as controversy....

[John]

Re: Brake fluid

As someone said, this is one of those "forever" threads with no end point. However, the OEM end of the business (which I'm involved in)would have settled on DOT5 silicone fluid years ago if it had a real benefit and had been proven to be beyond the attack of the product liability plaintiff attorneys; in truth, it has not - it's extremely expensive, the brake fluid processing equipment in assembly plants (high-vacuum evacuate-and-fill in less than a minute)can't handle it without entraining minute air bubbles, reduced atmospheric pressure and increased density altitude cause even more expansion of microscopic entrained air bubbles (the "mountain" effect on a sea-level bled car); it simply doesn't meet high-volume production criteria, although it's great in race cars that need the higher boiling point resistance but which also get regular thorough maintenance. Glycol-based DOT3/4 fluids have been around for 80 years, are well-understood by everyone, are very inexpensive, require no operator cautions about high-altitude effectiveness, and don't aerate under vacuum and pressure fill equipment cycling and ABS cycling. Their only drawback is the hygroscopic tendency over time which everyone has understood for 80 years - but they will NEVER put you in a position of having no brakes from one day to the next or from one altitude to the next, barring an actual physical failure in the brake system itself. The OEM manufacturer can't possibly control the conditions under which the car will be operated (or how much maintenance it gets in the real world), so they go with "what works" in a primary safety system which can get you killed if it fails to function properly when you need it - same reason you don't see composite steering arms and racks replacing steel and aluminum. It's that simple.

John '57 270

[John]

Re: Brake fluid

As someone said, this is one of those "forever" threads with no end point. However, the OEM end of the business (which I'm involved in)would have settled on DOT5 silicone fluid years ago if it had a real benefit and had been proven to be beyond the attack of the product liability plaintiff attorneys; in truth, it has not - it's extremely expensive, the brake fluid processing equipment in assembly plants (high-vacuum evacuate-and-fill in less than a minute)can't handle it without entraining minute air bubbles, reduced atmospheric pressure and increased density altitude cause even more expansion of microscopic entrained air bubbles (the "mountain" effect on a sea-level bled car); it simply doesn't meet high-volume production criteria, although it's great in race cars that need the higher boiling point resistance but which also get regular thorough maintenance. Glycol-based DOT3/4 fluids have been around for 80 years, are well-understood by everyone, are very inexpensive, require no operator cautions about high-altitude effectiveness, and don't aerate under vacuum and pressure fill equipment cycling and ABS cycling. Their only drawback is the hygroscopic tendency over time which everyone has understood for 80 years - but they will NEVER put you in a position of having no brakes from one day to the next or from one altitude to the next, barring an actual physical failure in the brake system itself. The OEM manufacturer can't possibly control the conditions under which the car will be operated (or how much maintenance it gets in the real world), so they go with "what works" in a primary safety system which can get you killed if it fails to function properly when you need it - same reason you don't see composite steering arms and racks replacing steel and aluminum. It's that simple.

John '57 270