Practically Classics Forum

Hi any one, I am considering using silicone fluid in the hydraulics formy resto Jag Mk2, any one any experience of using this please? The suppliers wax lyrical, as you might expect but are there any real problems to take account of?

Which hydraulic component are you referring to?

Yes, I've got this in both the brake and clutch system in my Midget.

It works well, and the pros are that it doesn't damage paintwork, and it doesn't absorb water from the atmosphere and so doesn't need changing so often.

The cons are that it's not so "slippery" so seals can run a bit "stiff" in the bores of cylinders - on mine this wasn't a problem except on the clutch slave cylinder, which was at first reluctant to return. Also, you can find that it will react with seals that have previously been used with mineral fluid, so they can swell, also causing partial piston seizure. Solved by re-sealing the cylinders though.

Overall I'd go for it, (as I did) but beware of the above, it's not necessarily as straightforward as a simple swap.

Cheers

One word of caution regarding the use of silicone fluid: As Luxobarge stated, it doesn't absorb water from the atmosphere, but that atmospheric moisture, assuming quite reasonably that it will still find its way into the system will then exist as globules of water. Now, since water will turn to steam under braking, that can result in a sudden loss of the pedal action until the steam condenses again. It really doesn't take much water in there to cause loss of braking when that moisture cannot be harmlessly absorbed. I say "harmlessly" since the volume of water present in the hydraulics of a car whose fluid is changed every other year will be minimal and not enough to cause the boiling point of that fluid to drop significantly.
I've only met this problem twice in nearly thirty years of buggering about with motors professionally so chances are it's not all that common, but I feel that this should be mentioned as it makes silicon fluid perhaps less suitable for regularly-driven cars than a regular, mineral-based fluid and, if there's still just as great a likelihood of moisture entering the system, then the "long life" advantage of silicon isn't so relevant as only flushing out and refilling will get rid of that moisture, though depending on humidity in your location, possibly not all of it.

I do like the fact that it's harmless to paintwork though.

Forgive me but I would be surprised if a hydraulic system had water (or moisture) entering it unless it was badly flawed. As in leaking and thus allowing water ingress. It would be a pretty poor system if it was like this and possibly just plain dangerous.

Out of interest, what would be the best way of flushing out a hydraulic system that had previously had DOT 4 fluid in it in preparation for upgrading to silicon? Bearing in mind that all the seals would be replaced with new of course

kevanf1 wrote:Forgive me but I would be surprised if a hydraulic system had water (or moisture) entering it unless it was badly flawed. As in leaking and thus allowing water ingress. It would be a pretty poor system if it was like this and possibly just plain dangerous.

Out of interest, what would be the best way of flushing out a hydraulic system that had previously had DOT 4 fluid in it in preparation for upgrading to silicon? Bearing in mind that all the seals would be replaced with new of course

Correct. While the issue of water in a silicone system that John raises is theoreticlly valid, your point is valid too, the upshot being that actually it's very unlikely to happen. The difference is that normal DOT4 etc. fluids are hydroscopic - they will absorb water from the atmosphere, even if they have very little exposure to air, they will still do it over time. Silicone fluid won't absorb water from the atmosphere, therefore IMHO as you say, unless there's a serious problem with the system or water gets in from under-bonnet washing or something like that, it will probably stay totally dry for many years.

Flushing the system - only thing I can think of is to run some silicone fluid through it. I didn't do that to mine, as it had all new brake pipes, new flexis etc. and the cylinders were stripped and manually cleaned, with new seals etc. As I said above, IMHO you're taking a risk if you use the existing seals.

Cheers!

As previously stated silcone fluid is non hygroscopic so moisture will not effect the fluid. One of the key issues is any water in normal fluid not only changes the viscosity, it also changes (lowers) the boiling point. With brakes being a friction system, on cars that dont get much use the brakes are prone to sticking which increases the friction under normal running conditions (without brakes applied). So it is crucial to have fluid that will maintain its performance under potentially tough operating conditions.

I am intending on using silicone fluid on my H120 rapier when it all goes back together and luckily will have a complete new system so have no issues with flushing etc.

If anyone knows a good place to buy some i'd like to hear!

Thanks.

jpsh120 wrote:If anyone knows a good place to buy some i'd like to hear!

Thanks.

Frost do it, but as it happens I've seen it for sale in a couple of decent local motor factors too, so not hard to get.

I've only ever seen 'Automec' branded silicon hydraulic fluid but surely there must be others?

kevanf1 wrote:I've only ever seen 'Automec' branded silicon hydraulic fluid but surely there must be others?

Yeah, presumably there are others, but that's the only one I've seen too - that's what I used. Seems OK?

Cheers