Brake Fluid Dot 4 vs Silicon Dot 5

[Kevin]

I just wondered what experiences people have on this subject.
I know that Silicon Brake fluid does not absorb water over time unlike traditional brake fluid therefore in theory the risk of corrosion is removed and components will last longer and I know that in some applications it can give a softer pedal and can also be a real pain to bleed properly and of course it costs 3 - 4 times as much and doesn't damage paintwork.
But is it really as good as suggested I know it lasts far longer and that lots of Moggie brake issues are caused by the brake fluid not being changed often enough allowing corrosion etc to start and can you just change from normal brake fluid to Silicon without an overhaul of the braking system obviously bleeding the whole system, but is there a correct way to approach this.
The reason I ask is that mine is due for a change this summer and I dont know which direction to go in (confused & puzzled but what's new there) some input from Jonathon would be useful as it would be from a workshop point of view based on practical professional applications.

[alex_holden]

I'm happy with silicone. The pedal feels firm enough to me and I haven't had any more trouble than usual bleeding it. I used it in my Series 2A Land Rover too.

I've heard it said that you need to replace all the rubber parts in the system (including the master cylinder seals) when you change from one to the other, meaning it's only practical to change to silicone if you're doing a complete brake overhaul.

Incidentally the slave cylinders can still corrode and seize regardless of the fluid inside because the outer part of the piston is dry (unless the seal is leaking). I've wondered about putting a smear of grease on the pistons on assembly but don't know if it might cause other problems.

[Peetee]

I've heard that you can't use silicon with traditional rubber seals. Does it work the other way round too? I would imagine the rubber compound used in the majority of car cylinders is designed for silicon fluid. Certainly the rubber used in windscreen seals has the feel of silicon rubber - much firmer and stronger. Perhaps new cylinders are failing quickly because they can't cope with the old style fluid?

[MoggyTech]

Perhaps new cylinders are failing quickly because they can't cope with the old style fluid?

Highly unlikely as the majority of new cars still run with DOT4 Brake fluid. There has however been a subtle shift in DOT4 fluid, in that it is now mostly synthetic rather than mineral.

If replacement wheel cylinders are not lasting, it's either due to the fact that they are pants quality, or brake fluid is not being changed every two years, or better still every year.

If changing from DOT4 to Silicon fluid, a total brake rebuild will be required, including all flexi hoses and every seal in the brake system.

[Stig]

Feel free to tell me it's dangerous but I didn't change everything when I put silicone fluid in. OK, I changed all the front as I was fitting discs and refurbishing the master cylinder but I don't remember changing the rear cylinders at the time.

[Axolotl]

You don't need silicone fluid on a Minor. It can actually increase corrosion risk. See this extract from the Miller Oils web site on the subject:-

<<Silicone Brake Fluid (DOT 5)

Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water. However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids.

It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal. It therefore requires special design considerations in baking systems. Further, because it does not absorb water, any water remains as globules, which can pool in low spots in the system and cause corrosion. This water can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency.

DOT 5 fluids are not recommended for motor sport applications.

PAG - Poly Alkylene Glycol Brake Fluids (DOT 3, 4 and 5.1)

Glycol-based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces. DOT 5.1 has a higher specification still and is intended for fast road and occasional track day use.>>

Also, this article dispels some myths:- http://importnut.net/brakefluid.htm

Looks like those who have mixed silicone and PAG are on borrowed time.

[MoggyTech]

You don't need silicone fluid on a Minor. It can actually increase corrosion risk. See this extract from the Miller Oils web site on the subject:-

Good information which is backed up on the Car Bibles Site. Silicon brake fluid is a bad choice for most applications. The theory seems to be it reduces the need for brake system maintenance, which IMHO is a bad idea. It increases corrosion in some cases, will wreck ABS pumps in modern cars, and it's expensive.

[bmcecosse]

Yep - waste of money on a Minor - it 'may' have some advantages on a competition car with very high (ie red glowing discs) temperatures but absolutely no advantage on a Minor. And yes - as others have said - it would need ALL rubber parts changed. DOT 3 is fine for a Minor - just bleed some new fluid through once a year.

[Alec]

Hello all,

I'm firmly in the silicone brake fluid camp and would use nothing else on a road car.

ABS doesn't affect me as I don't have it, and correct me if I'm wrong but Millers do not make silicone brake fluid so are hardly unbiased? Certainly in twenty years plus of using it I have not had any problems with it, cost being a negative though.

Alec

[IslipMinor]

AP are one of the recognised authorities on brakes and their design, and they do not recommend silicone fluid of any description:

From their website - AP Racing do not sell and do not recommend using a silicone based brake fluid with any of its products. Biased? Well maybe, but I would listen to their advice.

I am in the process of replacing the solid discs with vented, and will be using their latest 5.1 fluid, which is PAG.

[Alec]

Hello richard,

Note:- AP Racing. Silicone fluid is not, it seems suitable for competition use.

We are discussing road use which is a different thing. From what I remember the Automec fluid I use is compatible with glycol based fluids, does not require new seals, hoses before use and the point raised about water droplets is bizarre, water droplets from where?

Alec

[MoggyTech]

water droplets from where?

Water can still enter the system from condensation inside fluid tanks, and even the wheel cylinders. It is mentioned in the article in the link below.
http://www.imperialclub.com/Repair/Brak ... efluid.htm

[Alec]

Hello MT,

the majority of that link seemed to be in favour of Dot5, except for ABS. As I said I'm quite happy with it after so many years of using it.

Personal preference.

Alec

[IslipMinor]

Alec,

AP do not recommend silicone for any application. They have been warning and advising of this ever since silicone fluids became available.

This is from the general AP information:

Our technical service department is receiving an alarming number of calls from motorists reporting problems with silicone fluids. AP Lockheed neither markets such fluids nor recommends their use with our own or any other braking system.

Virtually all the problems relate to long/spongy pedal, sudden loss of brakes and hanging on of brakes. They reflect certain properties of silicone fluids identified by us over many years and recently ratified in SAE publications, namely high ambient viscosity, high air absorption, high compressibility, low lubricity and immiscibility with water.

Interestingly Lucas Girling also have never made a silicone brake fluid.

Personal preference, as you say, but in the light of this I think that people should be very aware of these findings, before deciding to go against very definite recommendations from the acknowledged major suppliers, and knowledgeable reports such as the one linked from the earlier posting from Axoloti.

Not for me.

[Alec]

Hello Richard,

in all the time I have used it (20 plus years) on several cars I have not experienced the issues raised, i.e spongy or long pedal, difficulty in bleeding, nor have I ever lost the brakes.

As I said personal preference and a satisfactory service over many years.

Alec

[bmcecosse]

As far as I know - NO new car has silicon fluid. There really is nothing wrong with Dot 3or 4, and just bleed the brakes through once a year.

[Alec]

Hello BMCE,

I would doubt that they would, there is no cost benefit to the manufacturer, why use something that is more expensive when a cheaper alternative is quite serviceable?
The fact that the fluid needs regular replacement is unlikely to concern them?

Alec

[bmcecosse]

Absolutely - although if it really was better stuff I would have thought the 'high end' manufacturers would use it where cost is unimportant. But - as you say you have had no problems, and I have heard of others you use it and have no problems - then it's up to individuals to decide for themselves!

[MarkyB]

If silicone fluid is incompatible with ABS it would explain why 'high end' manufacturers don't use it. Even 'low end cars have ABS fitted now.
I've used it in my Minors too and never had a problem beyond getting it bled properly for a hard peddle.

[dunketh]

This may be very naive of me to ask but...
Are we saying that by changing fluid to Silicon based stuff we would 'theoretically' never need to flush the brakes in the life of the car?