Dot 4 Brake Fluid V`s Silicon Brake Fluid

[Kevin]

I have been wondering about the advantages of Silicon fluid, its main selling points are that it doesnt soak up moisture like Normal fluid and therefore enables the components to last longer as there is no water present, but it costs a lot more than Dot 4 I think between 4 - 5 times as much and can be a real pain to bleed and gives a softer pedal
My thoughts are that if we change the brake fluid as recommended every 2 - 3 years there really shouldnt be the corrosion issues as the moisture is being kept to a minimum and the problems only really occur because we dont change the fluid often enough on the pretext of low mileages and if it really was such a wonder product why dont modern cars use it ?
Any thoughts anyone.

[alex_holden]

if it really was such a wonder product why dont modern cars use it ?

Cynically, it would add a few quid to the cost of each car at no benefit to the manufacturer (it's not exactly a major selling point) and it would reduce the amount of money the manufacturers make later on from selling replacement brake parts.

[rayofleamington]

silicon products are banned in most car plants anyway - they have a rather undesirable effect if you try and paint on top of them, so a finger mark / mist in the air could ruin a car.
If you've seen a brake fluid filling rig in a car plant, you'll notice how much of it ends up atomising / spilling / leaking if the rig or car is faulty!
Even if it could be used in a car plant, they might not use it - car dealers make a lot of money from servicing, so there is no incentive to make the car more expensive and reduce servicing bills. (which is rather different from the service interval - a bigger service interval is a positive selling point)


Anyway - this year I replaced the entire braking system on the series 2 traveller. At the same time I used new shoes. This was filled with silicon fluid and although I took a lot of care (e.g. fully priming the rear cylinders before fitting) it bled fine.
The only problem I got was the new shoes caused a soft pedal but that was ok when they were bedded in.

Although silicon fluid is compatible with EPDM seal materials, it is NOT always compatible with old seals that have been soaked in brake fluid and well used / heated cooled etc...

Seal material changed when in contact with fluid, so for the best robustness you should use new seals throughout. I used complete new cylinders and pipes throughout, but mainly that was because I'd robbed the entire braking system already to go onto another car!

[nebogipfel]

My only gripe with mineral fluid is it's a bugger if you get it on cellulose paint.

Many years ago when I were a lad, I'd just painted the front wings on a Fiat 127. I had the bonnet up and was in the process of fitting new brake pads I was just pushing the caliper pistons back in .............the fountain of fluid which came out of the breather hole in the master cylinder cap was spectacular - only snag was it landed on the O/S wing .......it really does go straight through fresh celly

Back to the point, FWIW I stick with mineral fluid, it's cheap, tried and tested, you can buy it anywhere and the problems are only apparent after many years of neglect of a braking system.

Silicon fluid is expensive and really must only be put in a brand new clean braking system.

[rayofleamington]

BTW - Brake fluid is NOT mineral based! If you use mineral fluid in the brakes you will wreck all the seals ;-)

[nebogipfel]

BTW - Brake fluid is NOT mineral based! If you use mineral fluid in the brakes you will wreck all the seals ;-)

OK, But you know to what I refer

As far as I'm aware it is often referred to as "mineral" but as to it's chemistry I know not!

I do know it's smelly, feels horrible on your hands, strips cellulose very effectively but has given me very little trouble as a brake hydraulic fluid over the years

[nebogipfel]

Apparently conventional fluid is glycol based "well you live and learn" as my old dad used to say

Back on the original question there is a short useful article here which might offer some guidance ......

http://www.xks.com/jaguar/news/techNote ... Fluid.aspx

[rayofleamington]

Unfortunately some of the technical comments in that article are 'over-simplified' (unfortunately I used be be a development engineer working on vehicle based brake fluid systems)

When brake fluid gets hot, the water vapour does not 'evaporate' as it is inside the brake lines. It stays there and is added to with new water over life.
This is why the service details of new cars state to change the brake fluid every 2 years. Over time the brake fluid absorbs water from the air in the reservoir (or water that permeated the hydraulic hoses and brake seals. As the % of water increases the boiling point of the brake fluid decreases - down to the range of normal working temperatures of hot brakes!! In a severe situation it gives the same effect as having a brake system full of air. o:
Ignoring the corrosion influence this is reason enough to regularly change your brake fluid.
2 years is based on an over-cautious approach. The chances of getting a lot of water into the fluid in 2 years is very low, but with the hundreds of millions of cars out there, it is hard to calculate the actual risk therefore they play safe on paper.

Dissolved air in a fluid does not expand in the same way as a staight gas - that is also not strictly accurate. However I can't remember the exact theory so I won't try and guess it - but the term is 'bulk modulus' - i.e. how squashy the fluid is.

The main problem with bleeding silicon fluid is (as far as I've been able to see) is that the skin formation (surface tension) is very different, making 'the wrong kind of bubbles' which are very hard to bleed out.

When bleeding Dot4 brake fluid it is fairly normal to see lots of fine bubbles. When bleeding silicon you don't see this effect.
Having heard lots of nightmare stories about how hard it was to bleed a Minor with silicon, I was surprised to bleed my Traveller without any complications! However - you also hear stories about people not being able to bleed a Minor when using Dot4, so it's not easy to know what relates to the fluids and what relates to the assembly and build procedure.

Eek - sorry for slippling into rambling mode!

[Alec]

Hello all,

I am one that changed to Silicone brake fluid a long time ago and have not found any drawbacks bleeding or with a 'soft' pedal. The American site says that it is not compatible with conventional brake fluid, (I'm not sure about the DOT spec) but Automec state that it is and I'm sure such advice would be correct.

I for one, am happy to pay the price, if only not to experience the feel of conventional brake fluid on my hands.

Alec

[nebogipfel]

The American site says that it is not compatible with conventional brake fluid, (I'm not sure about the DOT spec) but Automec state that it is and I'm sure such advice would be correct.

Ray is clearly the man to ask on this subject But I think they are referring to never mixing the two types in one system.

There is a great deal of opinion on the subject of fluids on the net and elsewhere and it is interesting to note that conventional fluid is still much more widely used.

FWIW Automec sell pure copper brake pipe and in my opinion the advice to use that on car braking systems is very unwise, especially for DIY fitment.

[Alec]

Hello John,

"FWIW Automec sell pure copper brake pipe and in my opinion the advice to use that on car braking systems is very unwise, especially for DIY fitment."

Why do you say that?; again the use of copper brake pipes makes the Americans recoil with horror.
I have made up and installed copper brake lines on a number of my cars over the last 20 years. Apart from the need to fix the pipework at shorter spans than steel, it is easy fit and also to dismantle many years down the road without shearing off a brake line as often happens with steel.

Alec

[nebogipfel]

Hello Alec,

Yes fitted properly and well supported copper can be OK. (I would never use it )

The problem with copper is that it work hardens very quickly and becomes very brittle.

I have seen two failures occur with copper pipe

1. The union had probably been overtightened (an easy mistake for the less experienced) which had stressed and hardened the flare on the end of the pipe which then snapped off in use - some vibration movement probably finished it off.

2. Pipe cracked - mainly because of vibration hardening when it was fiitted with a longish run unsupported.

Copper can be OK if fitted properly, hence my comment about DIY repairs.

Why use copper? The only possible reason is it looks pretty when polished.

Copper/Nickel alloy pipe has all the corrosion resisting properties of copper and none of the fragility of pure copper. OK it isn't quite so easy to work with, but it is safe ......... I know which I prefer.

[RogerRust]

I'm replacing all of the brakes including the master cylinder. I'm planning to use silicon fluid. I will fill up the wheel cylinders before I connect the pipes any other suggestions?

[rayofleamington]

I will fill up the wheel cylinders before I connect the pipes any other suggestions?

My only suggestion is to combine this activity with using 'bedded in' brake shoes. New brake shoes tend to give a rubbish pedal as they don't fit snug to the dums, so the steel part of the shoe flexes under load.
Having new shoes in conjunction witha complete brake rebuild is definately only for the massochistic!

[alex_holden]

I completely rebuilt Fenchurch's brakes (new cylinders, pipes, and shoes) and filled the system with silicone brake fluid, then took her for her MOT without having the opportunity to bed it in first. She passed first time. Just call me Mr Masochistic ;)

[Kevin]

Right so it seems that the only time it should be used is when the whole system is renewed then.

[57traveller]

That's what I did Kevin, using silicon fluid only after renewal of all the cylinders and seals etc. It's been in the saloon for 7 years and the Traveller for 5. However the Traveller fluid has had some renewal after replacing all the front wheel cylinders recently.
Automec do state that mixing with DOT 4 is acceptable but resulting in the silicon fluid advantages being nullified.

[Alec]

Hello John,

"Why use copper? The only possible reason is it looks pretty when polished. "

The reason was that the choice at the time was copper or steel. (This was 20 + years ago). I have subsequently used it as recently as 7 years ago. I can't remember if I was aware of Cupro nickel then but I had had no failures with copper so I stuck with it. (Who polishes brake lines?)

Copper does work harden but it does not go brittle, and is easily annealed if required.

Alec

[nebogipfel]

(Who polishes brake lines?)

Copper does work harden but it does not go brittle, and is easily annealed if required.

Alec

I think we're gonna have to agree to disagree on this one.

I was fitting Kunifer pipe at least a quarter of a century ago (making my own pipes, not ready made)

Having seen it fail, I would never fit copper, just an opinion offered - feel free to ignore it

If one person uses copper/nickel instead of copper as a result of reading this, I'm happy

Yes you can anneal copper but Mr DIY mechanic isn't going to do it in his driveway

Lots of show cars have polished copper everywhere

[Kevin]

Lots of show cars have polished copper everywhere

That must be the Panda cars then