Silicone Based Brake Fluids

[minstix]

I changed to silicone brake fluid in my brake and clutch systems on my MK2 Cooper S in 1990. It had become a popular choice at the time and it was claimed that you could put it in a system that had had traditional fluid in it.

You needed to pump as much of the old stuff through and discard it and fill it with the silicone fluid, pump it through and all would be fine. I understand it's not recommended these days to do that but that was then. I think I bled through about a litre of the silicone fluid before I was happy that almost all of the original stuff had been removed. Now I suspect you may not ever remove all of it this way. However, I did this and drove the car happily for the next two or three years, only covering maybe a couple of thousand miles a year during fine summer weather.

At the time of changing to Silicone fluid, I had changed the entire rear brake cylinders with new from Mini Spares. I did notice that the brake pedal was a little softer after the change and had read that this could be the case. It passed an MOT though like that.

In 1993 I took the car off the road and put it into storage. It lay untouched for many years in my mum's garage. I hadn't drained any fluids or done anything like that, just put it in there and to be honest forgot about. It wasn't until about six years ago that I decided to do something with it. Now, at last, I have a house with a decent sized garage, so the full restoration began.

I'm about 90% of the way through the restoration now. I've replaced almost entirely the brake and clutch systems. I removed the rear brake cylinders, which, like most other things, were completely covered in rust and so I renewed them with new cylinders that came from Mini Spares and contained all new inner parts, seals etc. What was interesting though is that on taking apart the old cylinders, the inside bores, pistons, rubber seals etc all looked just like new. No sign of deterioration and the fluid that came out of the system looked just as it did when it went in about 30 years previously. So that sold me on the idea of using silicone fluid in my newly restored system.

I've done some research into problems with silicone brake fluid. It seems that an issue can be that tiny air bubbles can get into it and are very difficult to bleed out, compared to normal brake fluid. Possibly impossible to totally remove them all. We are talking very very small bubbles. It's likely that this is the cause of the slightly more spongy brake pedal feel. I've been advised to fill the master cylinder very slowly and carefully and not from a bottle that's been shaken at all. It takes a long time for the bubbles to come to the surface.

One big bonus is that silicone brake fluid doesn't affect paintwork.

I wouldn't use it in my 'everyday modern car' that clocks up more than 18,000 miles a year and which is used all year round. However, for a car that I don't intend to race and will only cover maybe 2,000 miles a year and also be unused and stored in a dry garage for the large part of the year, silicone seems to me to be the right choice. It's horses for courses in my opinion.

I haven't completed my brake system at this time so haven't filled it yet but I will report back in due course.

Steve

[Peter Laidler]

Steve raises an interesting point about air becoming trapped in the fluid while it's on the shelf, in the garage. Not a thing that I;d whole heartedly agree with, because air ALWAYS migrates to its highest point (that's why it's such a bastard to bleed brakes when there's an air lock at the highest point!), albeit slowly, as Steve suggests, but depending on the viscosity of the fluid. But there is an answer to that conundrum that all of the old engineering apprentices out there will be aware of. And it's this....

With any workshop fluids that you use, it's always a good idea that I've practiced since I bled my first brakes. When you close the lid on the brake fluid, coolant etc etc, just squeeze the can or plastic bottle to expell some of the air before you screw the cap down. You've then got an atmospheric depression - or partial vacuum . of sorts - in the can. Nature being what it is, WILL draw air out of the liquid.

[minstix]

That's a good point Peter, squuezing of the fluid bottle before screwing down the lid is something I will practice in future.

I have an almost complete new brake system. However, I'm using the original brake servo, which has been completely cleaned out and restored using an original Lockheed kit of seals etc that I managed to get hold of. I've also restored the brake and clutch master cylinders with new seal kits. The bores were in very good condition, I guess sitting in silicone fluid the past 30 years may have helped there.

The outside of the two cylinders were looking a bit sad with surface rust marks. It was surface rust only but it didn't do much for their appearence (I don't like the shiny look of the new ones so decided to keep with the originals). I cleaned them and de-greased them and gave them a couple of coats of Eastwood Rust Encapsulator Platinum. This paint is a 'paint over rust or bare metal' paint and once cured it's very tough. Hard to scrape off even with a screwdriver. The other good thing about it is that its finish is remarkably similar to the look of the original Lockheed plating as originally done.

Bearing in mind that this is a 'paint', so another good reason to use silicone fluid. It doesn't affect Rust Encapsulator Platinum paint, I can vouch for that.

Steve

[steam]

I have used SBF in both new and used brake systems and there are no problems with either. I did follow the US Post Office advice and flushed out the whole system with meths when changing from regular fluid to sbf. In the new system I just filled, bled and drove. Simples.
And there is only one manufacturer 9f SBF , Dow Corning, but mny suppliers sell it under their own brand, its all the same stuff.

[minstix]

I've known of a few people who have asked if it's ok to use silicone brake fluid in systems where red rubber grease has been used to aid the fitting of new seals etc. Using a thin smear of it has for a long time been used to help lubricate the rubber parts when fitting on system that use traditional brake fluids.

There's no information one way or the other from the silicone fluid suppliers on this and that's often taken as 'it must not be used with our silicone fluid'. I don't thnk that's necessarily the case. It's more likely that no one has ever tried and tested it. So I thought I'd try a little experiment.

When I rebuilt my clutch master cylinder about three months ago, I replaced the piston and the rubber seals, all part of a new kit. The original parts showed no signs of wear or damage to be honest but I'm doing a nut and bolt restoration on this car so I replaced them anyway. I seperated and cleaned and dryed the old parts and then covered them in red rubber grease. Quite a lot more red rubber grease than would normally be used in fact.

I put the parts in a glass jar and poured new silicone brake fluid over them, covering them completely. I then put the lid on the jar and placed it out of the way and waited to see what would happen. Recently, three months or so after pouring the silicone fluid over the parts, I had a look inside the jar. The grease has largely disolved into the silicone brake fluid. Interesting. However, the rubber parts don't appear to have been affected at all at this stage.

Would this mix affect the performance of brakes? Interesting question. Maybe after a few months you could put new silicone fluid through the system, as if bleeding it in the normal way and so removing fluid that has rubber grease disolved in it.

I'll leave the test sample in the jar for a few years and see what happens and report back if it seems to have been detrimental to the rubber seals.

Steve

[Tim]

Sometimes you receive seals that already have a coating of what I assume is rubber grease, and pre-assembled wheel cylinders always seem to have some applied. I assume that they just use normal rubber grease, and not something silicone based.

Is there a recommended lubricant to use on new seals to be used with silicone brake fluid? Silicone O-ring grease maybe?

Tim

[Peter Laidler]

I've only ever used red rubber grease with silicon fluid from new and when I've just flushed out the old and refilled with silicon. 40 years old in my Cooper S and 24 years old in my last of the line year 2001 Rover Cooper Sport.

Both went through an MoT this year (not necessary with the 1969 Cooper S but he did the brake tests for free) and both sailed through. Users seem to forget that silicon and the old stuff simply will not mix. No matter what you do. A simple classroom experiment will show that within a few minutes, the two dissimilar liquids WILL separate.

You can over complicate this subject

[minstix]

That's a good point Tim about parts being pre greased. It's probably red rubber grease if it isn't brake fluid.

I used to use traditional brake fluid as a lubricant when assembling rubber parts on pistons etc but that was when I was using traditional brake fluid to fill the system. It's claimed that silicone brake fluid isn't as good at lubricating. I tried it when assembling some parts and that did indeed seem to be the case. So I used a very thin smear of silicone grease instead. The rubber seals went on easily with silicone grease.

I guess the silicone grease will disolve into the fluid in time. Silicone grease, like red rubber grease, doesn't affect rubber (some thin silicone oils contain a proportion of mineral oil to make it more fluid, so avoid them unless it states it's ok on rubber). I only used just enough silicone grease to help with the assembly of the parts.

My concern over using red rubber grease is that when I restored and rebuilt the brake servo, I did use red rubber grease. That was a while ago now, before I decided to use silicone brake fluid. I'm thinking that I'll use the car for a short while and then pump some new silicone fluid through to hopefully purge any dissolved red rubber grease from the system. I don't really want to have to take the servo apart again if I can help it.

Steve