Brake fluid change

[anchors]

I am considering changing the brake fluid on my 71 to silicone (DOT 5)fluid. I know the benefits of this this change but would like to hear from others who have experience with silicone regarding the pro's and con's of such a change. Thanks.

 

[SVO]

This is my opinion, and experience with silicon fluid. Others will probably disagree. On the positive side, because it's a silicon fluid, your rubber components will last longer. On the negative side, you have to be very careful of entrained air in the fluid, as it is difficult to bleed that air out of the system. Do not shake or foam the fluid in any way that adds air bubbles to the fluid. You will need to clean your brake system, making sure that all of old DOT 3/DOT 4 fluid is removed before adding the silicon fluid, as the two are not compatible. Your brake system seals will need to be perfect, as silicon seems to leak easier that conventional brake fluid. Based on my experiences with silicon brake fluid in a '78 model, I think that in the long run, one is better off staying with regular brake fluid and changing/flushing the brake system on a regular basis. I do mine every 1-2 years or when the fluid gets dark. Again, my opinion.

 

[anchors]

Thanks for your input, I'm inclined to agree with you on staying with traditional fluid. If it isn't broken don't fix it ! Thanks again.

 

[Rod75]

Just going to add to this post with some additional experience.
The P.O. of my Vette had used silicone brake fluid, and bragged of how great he thought it was. I didn't have much experience with it at that time, and proceeded to read on the subject.
The brakes worked OK, but seemed too spongy for me. I decided to bleed the system out. I started at the rear, and the first purge produced a fairly clean fluid. The next round gave some nasty color, floaters and impurities. A view in the M.C. showed real clean.
Off came the calipers and an attempted rebuild at the bench.
When I pulled the first caliper apart, I couldn't believe it! The back of each piston bore was loaded with bright orange rust goo and particles! The piston backs were the same. It was so bad that I decided to core them for rebuilts. The corrosion was working it's way around the piston sides and bore machined surfaces. Just not worth the effort to save.
This to me showed the effects of a poor flush job prior to silicone fluid intro. And the fact that the brake system is not impervious to ambient moisture penetration. The moisture was just pushed to the furthest point, and collected there.
A hygroscopic brake fluid would have most likely contained the moisture, and held corrosion to a minimum.
These moisture pockets in the caliper bores were like cushions, absorbing the action of the pumping brake fluid, giving that 'spongy' pedal feeling.
From then on in my garage, NO silicone brake fluid is used. I also purge/change brake fluids on all my vehicles every couple years, or when the fluid discolors.
My lesson learned.

 

[anchors]

One more nail in the silicone coffin. Thanks

 

[73shark]

A hygroscopic brake fluid would have most likely contained the moisture, and held corrosion to a minimum.

It's been my understanding that standard brake fluid being hygroscopic was the cause of the original problem because the retained moisture caused the bores to rust and pit and therefore leak.

A plus to silicone besides not attracting moisture and having a higher boiling point is that it won't ruin your paint.

 

[Peer81]

Have silicone brake fluid in mine for about 2 years. No problems so far. But i did rebuild the compleet systeem with new rvs lines prior to adding the dot 5.0. You also need to add it very slowly so there isn't any air in the fluid. Also no spong feeling or what so ever. Works great for me!

Greetings Peter

 

[73shark]

Forgot to mention that some spongy feeling in a non-power brake car can come from the rubber hoses.

 

[SVO]

It's been my understanding that standard brake fluid being hygroscopic was the cause of the original problem because the retained moisture caused the bores to rust and pit and therefore leak.

A plus to silicone besides not attracting moisture and having a higher boiling point is that it won't ruin your paint.

No, the cause of caliper bore pitting was not water in the brake fluid, but the lousy seal of the rubber boot to the caliper piston. Water would become trapped between the boot and piston lip seal causing pitting of the bore. Chevrolet finally half way addressed the problem in 1979 by recommending that the boot be sealed to the piston with a dab of silicone sealer. You can find that little note in the '79 Chevrolet service manual.

If you are using silicone fluid and are happy with it, drive on and save the wave.

 

[73shark]

I agree that the outer seal was part of the problem but most of the pitting on mine was on the fluid side of the piston seal.

Like everything, there's usually pros and cons.

 

[Rod75]

It's been my understanding that standard brake fluid being hygroscopic was the cause of the original problem because the retained moisture caused the bores to rust and pit and therefore leak.

A plus to silicone besides not attracting moisture and having a higher boiling point is that it won't ruin your paint.

I my case, not necessarily true. DOT3 glycol-based brake fluid starts to absorb moisture from the time it is installed in the system. It actually arrests the moisture and contains it. This property is desirable to help keep the moisture in check during service life. Moisture will enter the brake system, through microscopic pores in the rubber hoses and seals, or other introduced areas.
There becomes a time when the moisture saturation point becomes excessive, and will become more of a corrosion threat, not to mention lowering the boiling point of the fluid. And that is why it is recommended that brake fluid be changed at given intervals.

In my situation, the moisture was held in a localized pocket at the rear of the caliper bores. If the corrosion damage was caused by moisture over-saturated DOT3 fluid, evidence would have been through out the system, especially in the steel M.C.
This was not the case.
Silicone fluid repels moisture. And again in my case, had moved and trapped it in the bore bottoms. The top of my pistons were not corroded, just the bottom ends, where they created a pocket with the bore bottoms. Not to say that there was no moisture entry from the piston boots and seals, I see that as the most likely entry point.

Hey, if you like the proclaimed benefits of silicone fluid, use it. But for me, like the song sez: “Just one look, is all it took”. hnoes

 

[73shark]

Water lowers the boiling point of glycol brake fluid. That's the reason your not supposed to use it out of a container that's already been opened. That's in addition to the rust problem. BTW brake lines are terne coated to prevent rust.

I drove mine every day and the calipers started leaking in seven years w/ the original fluid. Replace the front w/ st-st lined and cleaned the rears. With silicone fluid, they're still good.

So as you said whatever works.

 

[Hib Halverson]

Like many things automotive, there are pluses and minuses with DOT5.0 silicon brake fluid.

First, there is no doubt that for classic Corvettes which are not driven regularly, silicon brake fluid is the best measure to reduce corrosion in the 65-82 disc brake system.

Downsides are:
1) If you use it, the entire system has to be flushed with silicone fluid first. DOT5.0 fluid and conventional brake fluids are not compatible.
2) Silicone fluid is much more difficult to bleed. Expect brake bleeding to take more time and more fluid.
3) Silicone fluid is not recommended for any duty cycles which put a lot of heat in the brakes, ie: road racing or track day events. The reason is that silicone fluid, when it gets hot, compresses slightly and that degrades brake feel. This is why no one uses the stuff in motorsports.

 

[73shark]

Thanks for the info.

I thought, in error I guess, that the higher boiling point made it better for race applications. Never too old to learn tho.

 

[Hib Halverson]

It is true that DOT 5.0 fluids have wet boiling points higher than DOT3/4 fluids but high boiling point is not all a racing brake fluid needs.

It also needs to not degrade pedal feel and that's why silicone fluid is not good in high-performance and racing applications.

 

[black_81_vette]

Its true that silicone brake fluid will not absorb water, but the fluid will float on top of water and if the water is at the lowest point in the system. IE the calipers,guess what happens next? You get rusty caliper pistions and a spongy pedal.

 

[Bob Heine]

I had silicone brake fluid in my 72 for 25 years (up on blocks). Most of the fluid seeped out of the calipers so before I put the car back on the road, I wanted to change to o-ring seals and stainless lines. Fluid in the master cylinder was clear with a slight purple tint. Fluid in the calipers was also clear but with the rust Rod75 described in his post. A system with silicone (DOT-5) fluid doesn't change color when water finds its way into the system and from my experience it doesn't deteriorate over time. If something is going on inside the brake system, there's no hint, even if you bleed the calipers. The only way to verify that your DOT-5 system is water-free is to disassemble it. Once you clean out the calipers, you get to hone your brake bleeding skills because as Hib pointed out, you are going to spend more time and fluid to get all the air out. If you have another Vette or daily driver with ABS, make sure you use DOT-3 or -4 because silicone will really mess up the ABS. If you have a pressure bleeder, make sure you thoroughly clean it before switching fluids.

I think this is what Rob75 was describing when he disassembled his calipers: "The back of each piston bore was loaded with bright orange rust goo and particles! The piston backs were the same."

 

[Rod75]

I had silicone brake fluid in my 72 for 25 years (up on blocks). Most of the fluid seeped out of the calipers so before I put the car back on the road, I wanted to change to o-ring seals and stainless lines. Fluid in the master cylinder was clear with a slight purple tint. Fluid in the calipers was also clear but with the rust Rod75 described in his post. A system with silicone (DOT-5) fluid doesn't change color when water finds its way into the system and from my experience it doesn't deteriorate over time. If something is going on inside the brake system, there's no hint, even if you bleed the calipers. The only way to verify that your DOT-5 system is water-free is to disassemble it. Once you clean out the calipers, you get to hone your brake bleeding skills because as Hib pointed out, you are going to spend more time and fluid to get all the air out. If you have another Vette or daily driver with ABS, make sure you use DOT-3 or -4 because silicone will really mess up the ABS. If you have a pressure bleeder, make sure you thoroughly clean it before switching fluids.

I think this is what Rob75 was describing when he disassembled his calipers: "The back of each piston bore was loaded with bright orange rust goo and particles! The piston backs were the same."

Hey Bob, thanks for the post and the pic! That sums up my description to the 'T'! I had a lot more corrosion in the bores to the point that the bore bottom wasn't definable.
This just reinforces the benefits of DOT3 & 4 brake fluid.
By the way Bob, I've been following your threads and I think you do amazing work!

 

[Bob Heine]

Hey Bob, thanks for the post and the pic! That sums up my description to the 'T'! I had a lot more corrosion in the bores to the point that the bore bottom wasn't definable.
This just reinforces the benefits of DOT3 & 4 brake fluid.
By the way Bob, I've been following your threads and I think you do amazing work!

Thanks Rod,
I love working on cars but luckily I never had to make a living doing it. The older I get, the slower I get but I keep trying new things. Last year it was welding and hopefully this year it will be airbrushing.

 

[South]

Hi Guys....when I restored my car I replaced all brake componants and even fitted copper-nicol pipes. I then used silicone because I was told that it prevents rust and it wont damage your paint. The thing is that I do have leaking brake sylinders....maybe because my car did not move for more than 18 months or I am not using cups made from the correct rubber. My main problem now is that the silicone fluid leaked into the brake drums and the brake shoes was "standing" in the fluid. Can someone please advice what I must use to clean the brake shoes with because it is now very slippery and not effective on braking.
Thanks South