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Converting to silicone brake fluid


sulzerman
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2500 Saloon

Fitting a recon brake master cylinder soon, so may convert to silicone fluid. Do I need to flush the system with something?

I have had silicone in my spitfire for 8 years with no issues and no paint stripped bulkheads either!

thanks in advance

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When I rebuilt the Spitfire I used silicone from the word go. On its first MOT the tester did ask what the fluid was (transparent reservoir) as he had never seen that colour before. Since then I've attached the lable that comes with the fluid saying that it is silicone.

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Here is Buckeye Triumph's in depth appraisal on converting to silicone.

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d01933a9c74c30001818fe4/1560384315450/Selecting+Brake+Fluid.pdf

The two fluids don't mix and just flushing one out with the other works. But, blow out the lines beforehand if you want.

Silicone does tend to retain air bubbles and I had to bleed my clutch 3 times over the space of a week.

The problem with rubbers is this, traditional fluid interacts with the rubber and mask deterioration. Silicone does not, so if your rubbers are deteriorating and you upgrade, suddenly you may have a problem. My rubbers were 200 miles old so fairly new and I had no problems upgrading.

Doug

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I flushed all the pipework through with meths when I coverted to silicon fliud many years ago. But I did put all new seals in beforehand. There were/are some people who say that you should put all new seals in if converting to silicon as the 'swelling' agents' within the fluids are different and reusing old seals is not advisable. I don't know if this a valid concern, based on scientific fact or not, but I prefer nor to take the chance when it comes to brakes.

Gav

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Another poorly investigated area with brake fluids is -

Both DOT4 and DOT 5 have additives to help them do their job.

The additives for one will be very different from the other albeit doing the same job.

One such additive is to allow the rubber to swell very slightly.

So if you use DOT 4 your seals will swell. If you change to DOT5 and retain the seals will the new swelling additive cause more swelling.

AND do not forget about the rubber pipes.

 

Roger

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There is another advantage to silicone DOT5 fluids that's not often mentioned.

DOT4 traditional glycol ether fluid is hygroscopic, attracting moisture. I've noticed on classics that aren't used often, brake cylinders can fail due to corrosion, which is probably accelerated by moisture.

When I switched to silicone fluid, which repels moisture, brake cylinder leaks became very rare.

Nigel

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27 minutes ago, dougbgt6 said:

Mr Lewis has a gizmo that tells you how much water you have in your fluid. He regularly lines up Herts and Beds on club night for fluid inspection.

Doug

I bought a brake fluid tester in eBay for about £10 and it appears to work fine on the DOT4 fluid in my modern cars.

Mark one eyeball is also a good test... Pale straw colour and the fluid is okay. If it's gone brown in the master cylinder reservoir, a change is overdue. The fluid colour correlates roughly with the moisture reading on the tester.

Nigel

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One thing to remember is that silicone fluid floats on water, which means that any moisture that does get in the system sits at the bottom of calipers and cylinders.

whe bleeding remove the calipers and invert them, then a couple of pumps will remove the water.

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10 hours ago, thescrapman said:

One thing to remember is that silicone fluid floats on water, which means that any moisture that does get in the system sits at the bottom of calipers and cylinders.

 

That also means that any microscopic water that gets into the MC, through the little hole in the top, will sink to the bottom of the MC close to the pipework fpr the system.

It then migrates, helped by gravity, to its lowest point.

Roger

 

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According to both Automec (who make the most readilly available DOT5 fluid) and if I remember the US Department Of Transportation (the "DOT" in DOT5, etc) DOT5 silicon fluid is 100% compatible with both 3 and 4 (though I have half a memory NOT 5.1?).  As such you don't NEED to do anything to swap from one of the other but...

1. While you can just mix them together they don't combine in any way, so you end up with brake fluid that loses a little pedal feel, due to the silicon but still absorbs water and can strip your paint.  Blowing through the lines will more or less solve that.
2. While you don't need to change your rubber seals they WILL fail at some point, regardless of the fluid you use.  Unless you've only just done them/fitted new parts what better time to do them than when you've already drained all the old fluid from the system/before you fill with new?  I mean you don't want to swap fluid, only to get the old "Read brake cylinder leaking" note on your next MOT certificate and start all over do you...

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What's the difference between DOT 4 and DOT 5.1 brake fluid ...

 

and before you ask  

What is Magura royal blood?
Royal Blood. All MAGURA brakes use low viscosity mineral oil (Royal Blood) as hydraulic medium. Contrary to DOT brake liquid it irritates human skin nor damage the paint off your frame. Biggest asset: It does not absorb water and actually lubricates the whole system. ... For all MAGURA disc and rim brakes.
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