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Dot-4 or Silicone.


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Hi I have tried to do a forum search but did not have much luck, so my question is below.

I am in the process of fitting a new clutch master cylinder and new clutch salve cylinder, which clutch/brake fluid should I use, Dot-4 or Silicone.

I know one benefit of Silicone, it is non hydroscopic.

I only use my TR6 occasionally so no racing.

Also is one easier to bleed than the other.

Any help would be much appreciated. 

Thank you,

Mick.

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37 minutes ago, mhossack said:

Hi I have tried to do a forum search but did not have much luck, so my question is below.

I am in the process of fitting a new clutch master cylinder and new clutch salve cylinder, which clutch/brake fluid should I use, Dot-4 or Silicone.

I know one benefit of Silicone, it is non hydroscopic.

I only use my TR6 occasionally so no racing.

Also is one easier to bleed than the other.

Any help would be much appreciated. 

Thank you,

Mick.

You are going to get a lot of opinions with this question. Some factual and some hearsay. The only real fact is both should work and silicone will not ruin your paintwork if the master cylinder works. 

For the record I have used silicone in all my rebuilds for the last 15 years and all seems fine. I also use DOT4 in my saloon as I can be bothered to change at the mo and it also works fine, although it did remove the paint from the bulkhead when the master cylinder leaked. 

Tim

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Hi Mick,

I know it may be senseless to tell this,

but another look on the question could be interesting.

Finally you must decide yourself.

Water goes where the water concentration is low.

And this works through rubber, seals, through (not all) pastics – and through the bleeding hole.

This happens independent of the ability of the liquid to fix the water or not.

It is well known the water causes problems like rust and boiling (brake fault) when hot.

With the hydraulic brakes in the 1920 a hydraulic liquid has been needed to fix (!) the water

which you can’t keep out of the hydraulic system because the water concentration is low there.

This is why the hygroscopic DOTs 2, 3, 4 have been created, you have to change them before they can not fix more water.

To be hygroscopic is their wanted function (their benefit) and not their fault.

This is why I'm conviced to use DOT4 (bought the car mit DOT5), and this without problems.

DOT5 has been created not to fix no water for any reason I don't know?

I don’t know anyone by person who had ever braking problems because of using DOT2-3-4 or DOT5.

So the discussion may be overrated.

Ciao, Marco

Edited by Z320
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Silicone DOT 5.0 -

Negatives:

1)pedal sponginess because of high gas solubility.

2) immiscibility with water means that any water migrates to the lowest point where it can't be bled out and causes corrosion and loss of brakes by boiling at only 100C (DOT4 ~200C), open bonnet in rain and water will get in when a raindrop hits the reservoir vent.

3) if you have a servo and it suffers an internal failure the silicone fluid will get sucked into the engine and destroy compression by being burnt to form sand in combustion chambers leading to ring damage and failure.

Positives:

1) it's not hygroscopic so it doesn't 'suck up' water. Meaning it's great for stored vehicles.

2) it has a very high flash point meaning it won't start a fire when splashed on a hot exhaust in an accident (Dot 3/4/5.1 will).

3) it doesn't damage paint.

 

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12 hours ago, ctc77965o said:

Silicone DOT 5.0 -

Negatives:

1)pedal sponginess because of high gas solubility.

2) immiscibility with water means that any water migrates to the lowest point where it can't be bled out and causes corrosion and loss of brakes by boiling at only 100C (DOT4 ~200C), open bonnet in rain and water will get in when a raindrop hits the reservoir vent.

3) if you have a servo and it suffers an internal failure the silicone fluid will get sucked into the engine and destroy compression by being burnt to form sand in combustion chambers leading to ring damage and failure.

Positives:

1) it's not hygroscopic so it doesn't 'suck up' water. Meaning it's great for stored vehicles.

2) it has a very high flash point meaning it won't start a fire when splashed on a hot exhaust in an accident (Dot 3/4/5.1 will).

3) it doesn't damage paint.

 

when dry or with 3.7% water contamination DOT 5 has a higher boiling temp (260 oC dry 180 oC wet) than DOT 4 (230 oC dry 135 oC wet)

But take the point that if water pools in DOT5 then it is closer to 100 oC

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1 hour ago, peejay4A said:

To complicate the discussion a bit, I use DOT 5.1. 

Which is a good performance fluid, though for best results should be changed every two years instead of three for DOT4.

Stuart.

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Given that it is for your clutch the boiling point is largely academic. For your brakes, the impact of condensation causing water to collect in the calipers can cause issues with hot performance.  The solution to this is get your brakes really hot by repeated braking from high speed and bleed with the theory that if you get your calipers above 100 C you will bleed out any water vapour present.

As for the rears you are unlikely to get those hot enough and the bleed nipple is at the same level (unlike the calipers where the water would pool low down)

Are gasses more soluble in it or is it that air mixed in it can prove tricky to bleed? When using silicone brake fluid in the brakes on a system that is empty, I fill, bleed and then bleed again the next day. Take for a blast to get hot and bleed again. If you are doing this to a new set up you can collect the fluid, run through a filter paper and re-use as my practice is wasteful.  (Don't do it if the system contains old fluid). You won't need to do this with DOT 4.

Before anyone says otherwise - the silicone fluid manufacturers say you can mix with DOT 4.  However doing so gets you the benefit of neither and the problems of both.

I have silicone in the 6 as I spent a deal of time removing the remains of leaked brake fluid when I resprayed the car - burned out what I could from the seams.

The 4a is still on Dot 4 as the paint does have a bit of damage from past leakage, so why pay more when the horse has bolted and have the more tricky bleeding?

 

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Old chestnut this one,

I love silicone brake fluid, been in there 6 years,  never touched it, however when I fitted it i did a really good clean of internal pipes,  replacment of rubbers etc 

I believe dot 1, 2 or 3 are like a thread release capillary action draws it down the thread ,  no leaks on your system come back, in 2 weeks oil leaking,  paint destroyed, sick feeling in the belly 

Pink x 

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Hi Pink

I have used silicone clutch/brake fluid years back in my Corvette C3, and the Corvette C3 brakes are a complete nightmare I spent hundred and hundreds of pounds on them.

The main problem was the calipers plus the hydroscopic brake fluid, please don't get me started on Corvette C3 brakes, good luck for me this is the wrong forum.

I have now decided I am going to use silicone  clutch/brake fluid.

I do not expect to much trouble because my clutch slave cylinder is a Past Parts refurbished unit, supplied by Richard Crewe-Read rcreweread@gmail.com thank you Richard.

I have been very lucky to get an NOS clutch master cylinder from TD Fitchett so no brake fluid to worry about here.

I intend to fit the clutch master cylinder first, and then pour in some brake fluid, letting it drain into a jar, which I will then use as my bleed jar, so hopefully all the old fluid should be gone.

Mick.

Edited by mhossack
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I converted my brakes and clutch to DOT5 a while back.  The claim of sponginess in the pedal with silicon fluid is somewhat overstated in my view, I didn't really notice any change  and my brakes are superb, although this can in part be attributed to other improvements made at the same time. 

A lesson learnt though was apart from flushing out all the lines with methylated spirits and compressed air prior to adding the new fluid (slowly does it as air bubbles are easily entrapped if you rush), CHANGE ALL OF THE RUBBER components including hoses!! I had previously stripped and repaired the master cylinder fitting a new piston cup.  I decided that since I had replaced it so recently, it should be OK and that I would leave it untouched .  That was a BIG mistake, it soon swelled up and stopped the piston from returning.  Once these items have been immersed in DOT3,4 or 5.1, they will react with the DOT5.

The main reason I changed was damage to pristine paintwork caused by a badly refurbished master cylinder and I didn't want to be faced with rectifying this again.  The other equally positive benefit was that the pistons in the drum brakes (and to some extent I guess the front calipers - although the pistons are chrome plated and less affected) don't suffer from moisture damage if sitting for prolonged periods (I believe the product was developed by the US Army for this reason).   I can hop in the car after a prolonged layup with the confidence that the brakes will be unaffected.

I wouldn't hesitate to do it again and for those who don't know, Harley Davidson motorcycles have used DOT5 fluid in their brakes for years!

Gavin

Edited by KiwiTR6
grammar
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The alleged sponginess with Dot5 is essentially down to rushing the bleeding process.

You just have to allow time for the fluid to settle and the air bubbles to come out of suspension and you will get a perfectly solid brake pedal.

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12 minutes ago, Andy Moltu said:

The alleged sponginess with Dot5 is essentially down to rushing the bleeding process.

You just have to allow time for the fluid to settle and the air bubbles to come out of suspension and you will get a perfectly solid brake pedal.

If you have a servo you wont notice the sponginess as much.

Stuart.

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Thank you all again very helpful.

Can anyone please tell me what that extension piece is coming out of the clutch master cylinder.

My car was an ex USA 1975 TR6, it was converted just before I purchased it, he did like a bodge, but the clutch master cylinder and pipes were all renewed.

I can only think it was a reducer because the fitting on the clutch line was the wrong size.

Thank you,

Mick.  

 

IMG_5596.JPG.679a8ad2bbf5799878cb40b19291b749.JPG

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Yes, it's a simple adaptator.

About silicone : take ultra great care, not all rubbers will accept silicone.

If you use rubbers who are not silicone friendly, you will have serious problems.

There is a good reason why some suppliers stick a label saying "do not use silicone fluid".

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The DOT specifications list the rubber compounds that brake fluids must be compatible with.  No reputable maker of rubber brake parts would use anything not on that list.

Many of the brake parts for these cars were designed at a time when the choices for brake fluid were mainly limited to glycol types and mineral oil types.  The DOT3 and later DOT4 specifications refered to glycol fluids.  Parts from this era designed for glycol fluid often came with markings like "Use only DOT3 brake fluid" (as opposed to mineral fluid). 

The design documents, tooling, and the rights to make and sell these parts have been bought and sold over the years, with most owners  not really interested in or capable of fiddling with the designs, so the markings remain. The outdated message also gets transferred to product packaging and literature.

Ed

Edited by ed_h
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