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I'm changing my brake fluid after a siezed rear caliper cooked my current fluid! (one of the problems of not using the car enough!) I have been recommended to use silicone fliud by a mate with an 1970's MG. Is this good advise?



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That depends.

If he is referring to 5.1 which is silicone but compatible with the older DOT4 and earlier minerals then it's fine.

If it's DOT 5 then..

Are you going to flush the whole system, clean/ replace (ideally replace) all the rubbers (including master cylinder) and change the fluid periodically to get rid of the water that will collect at the lowest points of the system?

If yes, then go for it. You'll get a fluid that's less prone to absorbing water and boils at a higher temperature.

If no. Then don't do it, you'll have a fluid where the water pools in the low points and so induces rust there (callipers?), it will for a sort of gel where it mixes with the old fluid (hence change rubbers) and so partially block flow of fluid.

The 5.1 still has the water pooling issue as it is not hygroscopic, thus does not absorb water, but water does naturally get into the system and so where the older stuff holds the water in suspension, the 5.1 doesn't.

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Is there any real benefit to using silicon-based brake fluid for road use especially with all the faffing about you may need to do to make sure the old stuff is out? I didn't think S1s were prone to brake fade under normal use.

S4 Elan, Elan +2S, Federal-spec, World Championship Edition S2 Esprit #42, S1 Elise, Excel SE


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I've heard 5.0 will trap microscopic air bubbles if you aren't very careful when adding it (pour it slow). As a result it can take longer to bleed if some air does get trapped along the way and needs time to purge. (pedal may be slightly softer for a while until all the air trapped along the way has "percolated" out) I've also heard that you're better off with conventional hi-temp stuff if you use the car at track days etc and silicone is more for garage queens that sit for long periods of time so the water doesn't corrode the lines and cylinders over time. Can't speak from a position of authority, and I haven't heard if any of this is true for 5.1, but I've heard the above said many times about 5.0.

Edited by comem47
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  • Gold FFM

5.1 and many of the new full-synths have boiling point over 500 degrees -

............Dry boiling point / Wet boiling point

DOT 3 - 205 °C (401 °F)/140 °C (284 °F)

DOT 4 - 230 °C (446 °F)/155 °C (311 °F)

DOT 5 - 260 °C (500 °F)/180 °C (356 °F)

DOT 5.1 - 270 °C (518 °F)/190 °C (374 °F)

Edited by Quikr

Lou Senko

Austin, TX

more, more, more....

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I have used the Ford Truck Heavy Duty DOT-3 for track days, but with the single pot small brakes (even with ceramic pads) it doesn't help much to prevent boiling. Brakes were a definite weakness on S1 and S2. I would try bleeding them but run a few hot laps and your done and then have to brake easier and earlier regardless of pumping to use the rest of your track time. (Watkins Glen turn 1 being an example) Of course everything changes on the street and you'll never see this boiling problem for the occasional quick stop or two, but the OEM brake setup still is lacking. I could see some large 4 pot calipers taking advantage of the space and larger rotors 16 inch wheels would provide you and would help dramatically vs the stock 14 in wheel setup.

Here's a quote from the web on the Ford Truck stuff:

Ford Heavy Duty DOT 3

550 deg F dry,

290 deg F wet.

Ford Heavy Duty DOT 3 is VERY inexpensive and ispopular among racers because of its excellent dry boiling point. Itabsorbs moisture quickly, but the racers don't care since they change theirfluid frequently. Comes in metal cans so it may be stored. I wouldnot use this in my Impala for the street.

Edited by comem47
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