Dot 5 silicone brake fluid

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#1 Post by **Western Thunderer** » Tue Apr 14, 2015 9:06 pm

Hi; just wondered if anyone has had any experience with Dot5 silicone brake fluid.
KVL has a sticker saying the hydraulic systems have been filled with it. The clutch master cyl is leaking, a kit is coming. I am aware that you cannot use dot 3/4/5.1 to top up the system or it causes mayhem.
i understand that silicone fluid is hygrophobic and does not absorb moisture as standard fluid does and as such is/was a popular mod for classics seeing very little use. A web search shows horror stories whereby any moisture that does enter the system sits as droplets causing localised corrosion or worse still if gets to a caliper can turn to steam under heavy braking with obvious consequences (& is I believe banned in Motorsport)
Now KVL is being used regularly I wonder should I replace all the seals and flexis and flush/convert back to dot4 or are any folks using dot5 happily with no ill effects?
Any thought welcomed!
Thanks,Andrew.

#2 Post by **Alec** » Wed Apr 15, 2015 5:51 am

Hello Andrew,

I've used nothing else for over thirty years and am very happy with it.

Alec

Polly · #3 Post by **Polly** » Wed Apr 15, 2015 8:25 pm

I too have used silicone fluid for more than 20 years, but I would not use it again by choice. It is more expensive, can be difficult to bleed, gives a hard pedal (if you can bleed it properly) and in the end, still needs to be replaced regularly.
I don't suggest you remove it, if you are already using it, but provided you don't spill the dot 4 on your paintwork, it has no advantages.

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#4 Post by **Western Thunderer** » Wed Apr 15, 2015 9:33 pm

Thanks both for replying. Reassuring comments.
I understand bleeding can be difficult and that the fluid must be left to stand before use as it aerates easily.
The clutch master is leaking and I'm about to put seals in it-clutches can be fun to bleed sometimes anyway,so fingers crossed this won't make it more of a trial..!
(Failing that I could do the slave too and go back to dot4..)
Anyway,thanks again-appreciated.

#5 Post by **david_lall** » Thu Apr 16, 2015 7:13 am

I've used silicone fluid since the early nineties in all my cars and would never consider anything else!

Polly · #6 Post by **Polly** » Thu Apr 16, 2015 4:34 pm

I do take your point David, and I'm not suggesting that you should stop using it. As I said in my previous post, I too have used it for many years, but my point is that it is more expensive, not so readily available, and has no advantage other than being kinder to your paintwork. On the other hand, it does have some disadvantages, not least its inability to absorb moisture in the system.
I first used it, under the impression that this was an advantage, that it would prevent corrosion, and that it would last the lifetime of the car. I have since learned that none of these are in fact true.
If we are to avoid corrosion of the system, it still needs to be replaced at regular intervals. But because it is not hydroscopic it does not remove moisture from the system, so even after changing fluid, moisture might remain.
I tried Silicon in my Pi clutch, but had to change to Dot4 because it simply wouldn't bleed properly.
I also still have it in my 2000 brake system, and most times it gives no problems. However I do have a strange problem that may or may not be related. Normal day to day driving I have no issues, but on longer trips, which usually includes towing a caravan, the brakes will work perfectly for the whole of the journey, however stop for a few minutes (like booking in at caravan site) and the brake pedal will all but disappear. Leave the car for an hour or so and miraculously the pedal is back. Bleeding the brakes several times have made no difference, but I suspect the problem is due to a bit of moisture in the system.
Of course I could be completely wrong, but I can only report as I find.

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#7 Post by **Alec** » Thu Apr 16, 2015 4:56 pm

Hello Polly,

I never change my silicone brake fluid, as I see no need. You refer to moisture, as it is not hygroscopic where is the moisture going to come from?

Alec

#8 Post by **david_lall** » Fri Apr 17, 2015 7:40 am

I would agree Alec. Also, using Gunsons Eezlibleed works a treat bleeding the brakes, but I can see no reason why Dot 4 should be any easier to bleed than Dot 5. Surely the clutch is just hard to bleed whatever fluid is used.

As regards to your brake pedal Polly, has it always done it? I would suspect the inner seal of the master unit as being faulty

Polly · #9 Post by **Polly** » Fri Apr 17, 2015 9:55 am

Yes David, as far as I know, it has always done it, but the specific set of circumstances don't crop every day, so difficult to be sure. Also the car is off the road at the moment, so even if I fix the fault, I can't be sure it's fixed.
Dot 5 fluid is a bit more difficult to bleed because it is quite a bit thicker than other fluids.
I don't see the logic in the faulty seal theory, for two reasons. First all seals were replaced before using silicon fluid. Now that's not to say the new seal couldn't be faulty, but I don't have a fluid leak,
and secondly, why does it only occur when everything is hot, and the car left stationary for a few minutes?
I have to admit, it's had me completely baffled, and it was only when the late John Cotterall it might be condensation and pointed me in the direction of several posts on the web to back up his thoughts. It's some time since I read the articles, but I seem to remember that dot 5 fluid does absorb some water, just far les than other fluids, and of course the system is not sealed to the atmosphere.
Perhaps we will never know if the theory is correct, but my plan is to replace all the seals and revert to Dot 4.
In the meantime, to revert to the original question, WHY use Dot5 fluid, when Dot4 is cheaper, and works equally well, if not perhaps better?

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#10 Post by **david_lall** » Fri Apr 17, 2015 2:36 pm

I also never change my DOT 5, and it poses no risk to the paint - that's enough for me!

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