synthetic brake fluid

[SR]

howdy folks, went down auto shop today ,they only sell this fluid, im thinking my pickup has the older stuff innit, is it just a matter of running the new synthetic through? , also cant find in my manual how much fluid in the system,quantity to buy,cheers steve

[rayofleamington]

Dot 3 or Dot 4 compatible is what you need. I would be very wary of using anything else unless the seals are new.

[SR]

cheers mate

[bmcecosse]

'They' say it makes old seals leak. Don't know - have never used it. I get Dot 3 in the pound shop !

[jonathon]

Silicon fluid should not be used if you have already exposed your system to Dot 3or 4. the materials are non compatible . A friend of mine who prepares rally minis has had a customer renew all of his brake system in preparation for the Monty Carlo Rally in the belief that silicon fluid would be an asset with regard to higher operating temperatures. Big mistake, only a few stages into the event, total brake failure, allegedly the culprit was the silicon fluid.

[Onne]

Is silicaon the same as DOT 5.1?

[SR]

i may be getting mixed up here, its all they sold down local motor factors, can i just get one with DOT 3 or 4 on it , cheers boys

[chickenjohn]

I think dot 3 or 4 is essentially the same stuff as in the Minor, thats what I've used to bleed the Traveller before the MOT and the brakes are as rock hard and work just as well as ever!

[bmcecosse]

The price will tell you SR - silicon fluid is very very expensive !

[les]

For what it's worth, I have had 5 years with silicon fluid in the brake system and have had no adverse effects, the components were all new at the time. Interesting to hear that brake failure was put down to the fluid, although if I was to be cynical I might say that in renewing all the system something else might have been overlooked!

[bmcecosse]

Oh yes -silicon fluid is the way to go IF everything is new. It doesn't absorb any water and so in theorey no more sticking pistons. And it does have very much higher operating temperature - mainly because of the no water thing. It just seems that seals which have already been in standard fluid don't like the change.

[jonathon]

Les,
The mini braking system in question was fully stripped to assertain the cause, as this car was one of the favourites to take honours in this event. The mechanic found complete failure of the rear brake seals, attributed to the fluid. Don't know the exact symtoms but can find out if you like.

[chickenjohn]

I think there is a good sound polymer/materials science reason for that, but I can't remember whAT it is as I did my chemistry degree 15 years ago!
But it sounds like a sound explanation BMcecosse.

BTW, did you get carriage for your engine parts sorted??

[les]

Well Jonathan, it would be nice to learn more about the seals, but I'm not trying to make an issue here, it just seems that many vehicles run this fluid without that outcome, hence my comments.

[jonathon]

I quite agree, which is why I will be asking for more info. I was not being critical of your comments

[Packedup]

DOT 3 and 4 can be mixed, and I imagine the original spec would've been DOT 3 anyway (don't have any books to hand, but it's highly likely). Anywhere should sell DOT 3 or 4, I think even Wilkinsons have it. Silicon stuff is better for not stripping paintwork, but I have read opinions that as it doesn't mix with water, any water in the system collects at the lowest points and corrodes away. Personally I'm not at all convinced by that argument, but it's not exactly hard to bleed the system every couple of years (as you should with mineral fluid).

It does react with old seals that have been exposed to mineral fluid, so it's best used after a full reseal and much flushing, as far as I know. Obligatory disclaimer, I am not a fluids expert (other than cheap beer ;) ) read up before working on your brakes, blah blah blah...

[bmcecosse]

If the seals had already been used with standard fluid that would do it - or maybe a 'shot' of old fluid was lying the pipes and got onto the new seals for a while before being bled off. . However - even new rear seals can fail very quickly on a Mini where left foot braking is being used extensively - and the pressure limiting valve has been removed or modified to allow it - and the brakes become stinking hot !! Lots of peeps use silicon fluid with excellent results - I have two tins 'on the shelf' - but not the enthusiasm to change all the seals.

[Chris Morley]

I'm surprised that no-one on here has mentioned the article in Minor Matters quite recently. A long time member used silicon fluid after a full restoration. Shortly after gaining an MOT his car ground to a halt with seized brakes (now do I know THAT feeling.... ). To cut a long story short, he proved that the brand of silicon fluid he used had swelled the NEW rubber seals. Then he asked a local Minor specialist to recommend another brand of silicon fluid which they swore by and had caused no problems. Result: no swelling of the seals. So it would seem that the make of silicon fluid used is rather crucial......

Sorry, I can't remember the brand of silicon fluid which was recommended - has anyone got a copy of that issue of matters?

[Stig]

Well I'll put my vote in favour of silicone fluid (silicon is a metal!), been using Automec stuff ever since I fitted discs with no problems at all. I did change the master cylinder seals at the time but not the rears as they were fairly new.
OK, it's probably twice the price of the normal stuff (glycol?) but it doesn't need changing every couple of years. Everyone does change their brake fluid regularly like you're supposed to don't they...?

[ColinP]

Well, actually, silicon is a "metalloid" - that means it's almost a metal.

It doesn't conduct electricity well (semi conductor), and it's in the same Periodic table group as carbon - definitely non-metal!

Both form polymers which are stable, and that's what the fluid is (usually silicon + oxygen to form a silicone oil - the viscosity depends on the length and structure of the polymer). To do that, one has to catalyse the polymerisation - that's where the differences will lie, because each company will have its own "favourite" initiators and terminators (to start the polymerisation and the stop it at the right point). It's also quite possible that the different polymer lengths and structure are what cause the problems....

Colin