Brake fluid

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TrevorG
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Brake fluid

Post by TrevorG » Tue Feb 05, 2019 8:05 am

Apologies if this has been tackled before.
My series 6a has a dot5 tag near the brake fluid pot.
I know that dot5 used to have a higher boiling point than dot4, but they all seem about the same now with dot5.1 being even higher.
Dot4 and dot5.1 are glycol based whereas dot5 is silicon based and is known to attract moisture.
Glycol and silicon should not be mixed, but do people recommend going to dot5.1 with flushing the system and how? Or is the silicon less harmful to seals etc?



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Brake fluid

Post by AntC » Tue Feb 05, 2019 8:19 am

I am a great fan of silicon brake fluid which does not absorb water as far as I am aware. I have always changed all the seals throughout the braking system when changing to silicon and, of course, the biggest advantage is that it doesn't strip paint :D


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Brake fluid

Post by philhoward » Tue Feb 05, 2019 8:30 am

Silicone fluid doesn’t absorb water but as such allows it to pool in the lowest point instead.

Changing between “normal” and silicone involves changing all the seals as they don’t mix at all.


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Post by Old and Slow » Tue Feb 05, 2019 9:36 am

Out of interest, and maybe this is a rhetorical question, but....
How can you check that the Dot 5 label is correct and the system is filled with silicon fluid?
Is it a different colour?
If it doesn't absorb moisture then presumably it will be non-conductive, but then fresh dot 5.1 should be non-conductive too.
Hopefully there is an expert on the Forum who can advise.


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Brake fluid

Post by philhoward » Tue Feb 05, 2019 9:41 am

Silicone fluid is generally purple I think?


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Post by Nick » Tue Feb 05, 2019 9:42 am

Stick to 5.1.


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Post by philhoward » Tue Feb 05, 2019 9:52 am

Forgot the other thing to remember with Silicone fluid - whilst it doesn't absorb water, I believe it does absorb air which can lead to a slightly spongy pedal.


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Post by Roger Pennington » Tue Feb 05, 2019 11:02 am

Given that silicone fluid is claimed to not damage paint, a simple test should be to extract some from the reservoir and drip it on your paint - if it doesn't strip it, it's presumably silicone. (use an inconspicuous area of course! :lol: ) Though like Phil, I think it's usually purple.
philhoward wrote:
Tue Feb 05, 2019 8:30 am
Silicone fluid doesn’t absorb water but as such allows it to pool in the lowest point instead.
And I've heard that because any stray water is present as "free water" if the temp. is high it can form pockets of steam, (something which doesn't happen if the water is absorbed into the fluid, as in normal fluid)
TrevorG wrote:
Tue Feb 05, 2019 8:05 am
My series 6a has a dot5 tag near the brake fluid pot.
Trevor, that does seem to beg the question - what is the current status of the brake system, has any work been done on it, or any fluid been added? - bearing in mind that the two types don't mix, and require a rubber change if changing over.


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Post by AntC » Tue Feb 05, 2019 11:33 am

My Jaguar Mk 2 which I restored in the early 90s has been running silicon brake fluid since around 1995 and apart from very occasionally topping up the level in the reservoir has shown no problems at all. I don't own it now and the new owner, a friend, drives it much harder than I did. For me, and if you are going to restore the complete braking system, it's a no-brainer.


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Post by roymck » Tue Feb 05, 2019 11:34 am

Easy test , conventional brake fluid can be mixed with water , silicone will not .



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Post by halfpenny » Tue Feb 05, 2019 12:33 pm

FWIW, in the 70's AP Lockheed did extensive texts on the merits of silicone brake fluid against the conventional stuff. There were sufficient disadvantages not to switch, and at that time it was definitely not recommended. But it works. Personally, I stick with conventional, but flush regularly to get rid of moisture, which is frankly not a big problem in our non-humid country, and rubber debris from the seals.



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Brake fluid

Post by TrevorG » Tue Feb 05, 2019 4:50 pm

Hi, sorry about delay....busy machining a large John Deere today!

Perhaps I should also have mentioned that the fluid in the systems (brake and clutch) is defo dot4 or 5.1 not 5. But the car has this plastic tag screwed to the bulkhead which is faded almost too obscurity and states Dot5 only. I don't think it's original so someone has put it there for a reason. I'm flushing both systems anyway as I have put new cylinders in at the back and calipers on the front but the masters seem okay until engine running, difficult to test vacuum assist.
My concerns were whether I need to do more flushing just incase.
Lastly, I got it wrong, glycol absorbs water, silicone repels, doh!



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Post by Roger Pennington » Wed Feb 06, 2019 12:21 pm

TrevorG wrote:
Tue Feb 05, 2019 4:50 pm
I don't think it's original so someone has put it there for a reason.
Definitely not original, so I suppose it does tend to suggest that the car has had it added to it's system at some point. If it's subsequently been changed back, then the question is what was done when it was changed back? If you've replaced the calipers and wheel cylinders that's two main areas, the servo doesn't have fluid in it, so it shouldn't be a problem, so the unknown is the master?


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Post by TrevorG » Wed Feb 06, 2019 1:15 pm

Thanks Roger, yes that's my thoughts....don't know when it was converted back to dot4 if it ever had dot5? I only know there is a tag. Therefore my question on whether others have converted and flushed? out the system.



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Post by halfpenny » Wed Feb 06, 2019 1:47 pm

If switching, or unsure, you will need to change all of the seals, including the servo cylinder (which does have fluid in it), and wash everything very thoroughly in brake cleaner or meths. The servo vacuum can should be ok.



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