Silicone brake fluid - good or bad?

Yup. Some people do have strange ideas. Many held by those who contribute to the uk.rec.cars n/groups. Mike.

: Of course if the first part of that was true, we'd never come across : rusty brake components where conventional fluid has been used. I've a : pile of Morris Minor brake cylinders that prove otherwise.

Do steel brake pipes rust from the inside?

Ian

In theory if steel is not in contact with air it shouldn't rust, should it?

Ron Robinson

: In theory if steel is not in contact with air it shouldn't rust, should it?

Not without some other source of oxygen. I was thinking that if wet brake fluid could cause corrosion inside steel cylinders it should also play havoc with steel pipework. Do steel cylinders rust in the bits that don't get exposed to air?

Ian

The more I think about it, the more I'm convinced that the rusting starts outside of the piston where there's air and enough brake fluid to draw in moisture. This then causes the piston to sieze, especially if used infrequently. The big question is whether silicone fluid prevents this from happening.

The 'dust' seal on the average piston is really no more than that. Contrast it with the proper seal at the end of a caliper bore. And any leaking cylinder I've seen has always more damage to the bore towards the opening than further down which suggests moisture etc getting in there rather than from the fluid.

Yup. But what I'm wondering is, do you get more moisture getting in there and causing rust when you use conventional fluid than when you use DOT 5? Even when the the piston seal is healthy, there's usually a trace of fluid on the bore - but does that fluid draw in more moisture? Perhaps an experiment is in order...

I became a believer in Silicone fluid when I ran an Alpine-Renault GTA. The rear brake callipers on these are aluminium (the same as on the front of the higher-performance Citroen BXs). There is a complicated handbrake mechanism involving a steel rod that is pushed via a snail-cam and that goes through the back of the calliper and pushes on the piston from the inside. These callipers always leak a tiny amount of fluid past the rod seals. This absorbs moisture and soon enough you get white powdery aluminium corrosion and a much bigger leak. I managed to get hold of two callipers in better condition, and pressure cleared the system of the old fluid, and refilled with Silicone. I had no further trouble in another 4 years of using the car.

Interesting article in the Morris Minor Owner's Club magazine (P16 Jan/Feb 2005) about someone having problems with silicone after it being okay in the past. On stripping the brake system they found all the rubbers had swollen by about 30%

They suspected it was the unbranded silicone they had tried so soaked one new seal in branded silicone, one in the unbranded and left a control seal in the air. After a month the seal soaked in unbranded silicone had swollen; the seal soaked in their usual branded source was fine.

Perhaps they have hit upon the cause of the polarization of people's views on the subject.

One other point, I don't think you are supposed to use silicone fluid in systems that have had the ordinary stuff without cleaning everything out thoroughly and renewing all the seals. Similarly I don't think the ordinary stuff should be added to an un-cleaned formerly silicone system.

I had a very interesting chat with someone at the NEC Restoration Show recently.

It seems the problem is with unbranded fluid. The well-known silicone fluid exceeds most technical standards for performance and is rated higher than most 'standard' brake fluids.

I was told that there is a reluctance on the part of the motor industry to adopt the silicone fluids to protect the revenue received for regular fluid changes carried out by the trade. Silicone fluid lasts indefinitely.

As a natural cynic, I'm not going to commit myself at the moment. There has also been some criticism of changes to the 'feel' of brakes with silicone fluid in them. Keen to learn more, I got myself some silicone fluid and will try it in my 1972 Jaguar XJ6 from the big spring service.

Anybody interested in my findings can read about them in Practical Classics magazine or online at

Martyn

"Martyn Moore" realised it was Fri, 21 Jan 2005

If that were true, why do modern cars go for 20,000 miles or more between oil changes?

Strange then that my car has 'lifetime' fluid in the auto gearbox, rear axle, and 15,000 mile plus engine oil changes. Some also have 'lifetime' coolant.

Silicone fluid may possibly last a 'lifetime', but since it doesn't absorb water or moisture, any entering the system could cause real problems. And if moisture didn't have access, there'd be not the same reason to change 'ordinary' brake fluid regularly...

In message , "Dave Plowman (News)" writes

That's not a logical argument. Ordinary brake fluid is hygroscopic, so it will absorb atmospheric moisture through even the tiniest air leak. Silicone fluid does not suck moisture out of the air, so all you have to ensure is that water cannot drip into the fluid. Good workshop practice will ensure this.

As I said a few weeks ago: ask anyone with aluminium brake callipers what sort of brake fluid they use. In my experience you'll find a large proportion using silicone, as aluminium callipers are notorious for corrosion problems with ordinary fluid.

The only problem I've found with silicone is that old indirect servos can get sticky. This may well be because of the old seal issues that others have mentioned.

It has nothing to do with leaks, or good workshop practice. The reservoir is open to atmosphere. Any condensed water in the reservoir finishes up in the brake fluid. No way to prevent it other than designing a sealed system. Mike.

"Mike G" realised it was Sat, 29 Jan 2005

Why should it? Silicone fluid is not hygroscopic.

In message , Mike G writes

Why should there be condensed water in the reservoir? Are you expecting the atmospheric air temperature rate-of-change to be fast enough for moisture to condense out in a plastic reservoir?

It doesn't mix with silicone brake fluid, but it still collects, and puddles in the bottom of the reservoir, as it's specific gravity is higher. From there it's soon carried into the pipes and brake cylinders. Mike.

Certainly. But speed has nothing to do with it. Just humidity and temperature. A humid day followed by a cold night, and water will condense on the inside of the reservoir. The fact that it's made of plastic, doesn't make any difference. It's exactly the same process as takes place within theengine itself, that causes mayo to form in the tappet cover and oil filler cap of cars that are used for short trips only. Prove it for yourself by removing and looking inside the cap of the brake fluid reservoir, or oil filler cap, first thing in the morning after a hot humid day. Mike.

In message , Mike G writes

I've done just that, and have never seen any moisture on the inside of a plastic brake-fluid reservoir! (Metal ones then yes, I have).

The air bleed to the atmosphere is a tiny hole, and as you don't normally take the brake fluid cap off more than a few times a year (with see-through plastic reservoirs), then the air space above the fluid isn't 'breathing' in and out, so there is no (or very very little) air interchange.

I'm not saying it couldn't happen, but I think it's very unlikely to be the source of moisture compared with the very hygroscopic action of conventional brake fluid.