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MG MGB Technical - Silicone Brake Fluid Yes/No ?

Approaching the final stages of a 2 year restoration of my 75b, including a new paint job. There was considerable damage to the paint where break/clutch fluid had leaked on the paint. Break and clutch master cylinders have been rebuilt. Thinking about replacing regular break fluid with Silicone Brake Fluid. Any strong feelings one way or the other? If Silicone is recommended any particluar brand? Thanks in advance for all your help.

George
G.L. Logan

YES! It functions well and won't damage the paint when you have your first leak 15 years down the road! The only problems with it are a slightly softer pedal and its a real pain to bleed the system. Also, don't use it if you have any old seals that were used with any other brake fluid.
Jeff Schlemmer

George,

If you are going to change to silicone brake fluid, you need to be sure that all rubber parts and seals in the brake system are new - from the master cylinder to the hoses to the brake cylinders and calipers, and that all of the old brake fluid has been completely removed from the metal brake lines.

Also, once you change, make sure you put a tag or some sort of label on the master cylinder so that no one by accident ever puts in regular brake fluid. You do not want to mix the two fluids.

When I was restoring my Corvette a few years ago I changed to silicone brake fluid and I am very pleased with the results.

Robert
Robert Browning

Unless you use silicone, you will have paint problems around the master assembly. Eventually. I can not vouch for the "soft pedal" quoted so often, since the pedal on my car is rock solid. Beeding has never been a problem either. Keep in mind that when you bleed any brake fluid, you move the pedal very slooooowly for best results.

If anyone can point me to some technical information on the results of mixing silicone and regular brake fluid, I would welcome the reading. My understanding of the DOT mark is that in order to attain it, the fluid has to be saflely compatible. I to parrot the wisdom of "New everything" but it is not based on any technical or chemical knowledge of my own

Pete
Pete

Pete - While this is not an official technical explanation, I have read that the absorbtion/desorbtion rates of DOT and DOT 5 fluids are not the same and therein lies the problem. The rubber components in a brake system do no completely clear themselves of the on fluid before absorbing the other (regardles of how small an amount is absorbed). The result is rubber components becoming soft or swelling. I have always completely cleaned the brake system, including the metal tubing and renewed all the rubber components before switching to silicone fluid and have never had a problem with it. Cheers - Dave
David DuBois

"If you are going to change to silicone brake fluid, you need to be sure that all rubber parts and seals in the brake system are new - from the master cylinder to the hoses to the brake cylinders and calipers, and that all of the old brake fluid has been completely removed from the metal brake lines."

I'm certainly no expert, but I completely disagree with that. I had a leak in the master cylinder on my TR6, and replaced it with a new one. At the same time, I swapped over to silcon fluid. All I did to make the change was to flush the new silicon through the system until it bled out clean looking silicon. I didn't replace any seals, or flush the system with alcohol, as some suggest.

bleeding the brakes was no problem and I had just as firm brake pedal as before. I drove the car for 8 years, many thousands of miles, with no problem at all until the replacement M/C failed on me - the only thing in the system that had never been exposed to anything but silicon.

FWIW
Dan Masters

Well, I went looking for an answer to my own question and did learn a few things.

The controversy around silicone is in the swelling (and hence sealing ) properties of the fluid. Glycol has a well known and predictable swelling ratio with the rubbers used in the automotive industry (there are three major rubbers used, with one being most common in todays brake parts.) If our new brake parts are being made with modern rubber parts, we will not have a problem. If you find a nice old stock master cylinder for your 67 MGB, it may not like the silicone fluid.

If you are nsure of what fluid is in the car, pull a bit out and add a bit of water. Silicone will do a separate layer with the silicone fluid on top. Glycol will go milky as it absorbs the water. Chances are if you have glycol more than two years old it will already be milky from the water absorbed from the air.

If you run a brake booster, there is a very good reason to avoid silicone fluid: In some wear/failure modes, boosters allow brake fluid to be drawn into the engine and burnt. When silicone fluid is burnt, it reverts back to silica. Silica is SAND and not good for the engine bores.

I have yet to find a chemical or technical explanation of why the two do not mix, and I will also confess to following in Dan's footsteps with no adverse effects.

On a new topic, has anyone tried the newest "Synthetic" fluid rated as DOT 5.1?

Pete
Pete

Thanks for everybodys response to my original question. Pete's research brings forth another question.

Pete my B is a 75. It has a dule line Servo Assist. Is this what you are referancing as a brake booster? This unit does have a vacuum hose that goes from the servo unit to the manifold. Do you have an idea as to what wear/failure modes would cause flude to be drawn into the engine.

Again thangs for your comments and help.
George

George:

I guess the best way to describe the failure mode is "A worn servo seal" separating the engine vacum from the opposite side. I have seen this happen on a big healey, and the symptom was a mysterious loss of brake fluid. There was no noticeable loss of breaking powe.

Pete
Pete

I asked this question a few years ago. The previous owner had switched to silicone fluid about 1989. When it was time to change my MC in 2003, I had heard how hard it would be to bleed if I kept silicone and other negative comments. I stayed with silicone fluid, and no problems, rock hard pedal for the past 3 years, and I really don't worry about my brakes very often.
Joe

I STRONGLY echo the comments about replaciing everything in your brake system before switching to silicone (DOT 5) brake fluid. I tried switching to silicone in my '76 B several years ago. While I flushed the system thoroughly, and replaced the M/C, I did not rebuild/replace the front calipers. After about 150 miles, the calipers started leaking. I have never, in 22 years of MGing had a caliper leak. Sieze, yes, stick, yes, leak, never!

I took the '76 straight home, replaced the calipers, flushed again, went back to DOT 4, and it was fine after that.

This does not mean I'm opposed to silicon brake fluid. It means that it is incompatible with brake components that have been been exposed to DOT 4. I'm not arguing with Dan Masters, but I might suggest that his experience is perhaps atypical.

I'm presently doing a ground-up restoration on a '65 B, and for that car, with all new components and lines, I'll review the latest wisdom regarding DOT 5 or DOT 5.1 before making my choice, but for my currently-used cars, it's strictly DOT 4 for me.

Incidentally, I do not have any difficulty with spilt brake fluid eating the paint around my M/Cs. I keep my DOT4 in a trigger-operated oil can so I can top up whenever necessary without spilling.

Beyond that, keep in mind that we're talking about your brakes! They're important. The main advantage of DOT 5 is that it won't eat your paint. If that concern compromises safety - even by the slightest amount - it is not worthwhile.

FWIW,
Allen
Allen Bachelder

This issue about brake servos (or boosters) does not apply to direct acting servos only to the indirect ones.
David Witham

In the early part of 2000, I cleaned up my engine bay, partly because the paint was damaged by brake fluid. I rebuilt the MC, and flushed the wheel cylinder seals to try to clean them up. I did nothing to the clutch system, except flush the system as well as possible (as I also did with the brake system) with silicone fluid, and refilled both systems with silicone fluid. There were no problems with bleeding either system, and the brake pedal is rock solid. I regularly slalom the car, and the brakes and the clutch have performed flawlessly.
In the 6 years since I did this, I have had one wheel cylinder and the clutch slave cylinder develop a leak. Also, the rubber seal inside the brake fluid reservoir cap has become all "wavy" and no longer makes a seal, although no fluid comes out. Maybe that was due to the silicone, or maybe it was because they were the original parts on a 30 year old car. However, my engine bay still looks great. So, thats my experience with switching.

Erick
Erick Vesterback

George, As has been stated, silicone won't damage the paint. That's not the important reason for using it. It doesn't retain or create moisture. Regular brake fluid is fine for a daily driver that is constantly maintained. It has been used for years and works fine. The kicker is, most of our antique and classic cars set idle a lot, some for extended periods of time. Regular brake fluid will absorb moisture over a period of time, especally while sitting, and create rust spots inside the brake system and sooner or later a leak will develop. Silicone doesn't do that. I've personally used it for years and wouldn't change back. As far as the "Soft" pedal theory, I have not found this to be true IF THE SYSTEM IS BLED PROPERLY! You just have to take your time bleeding the system. I bleed the system like I do aircraft brakes. Starting with an empty system, pump the fluid in from the botton up through the bleeding ports, never by pumping the master cylinder. It works for me. Paul
Paul Jennings

I certainly don't profess to be an expert on this, or any other, MG related subject, but here's my two cents worth. When I started restoring my '73 Roadster six years ago, I switched over to synthetic brake fluid. I used "Valvoline SynPower" formula brake fluid, specifically because it says right on the label: "mixes with and is compatible with conventional brake fluids". All I did was open up the bleed nipples and drain out all the old fluid, filled up the MC, and then bled my system with the new Valvoline synthetic. That was five-six years ago, and I have had no problems whatsoever. Sure, maybe you SHOULD replace all the rubber seals in the system, but that might be true no matter what type of fluid you choose to go with. As for a softer brake pedal, I've never driven my Roadster and then jumped into someone else's car to check for any differences. If the pedal IS softer, I can't tell. Hope this helps. Brando
Brandon Fox

"Valvoline SynPower" is a conventional DOT 4 brake fluid and absorbs water and 2 year changes are recommended. “Synthetic” is a marketing ploy.
Paul Wiley

This thread was discussed between 06/08/2006 and 09/08/2006

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