Jumped into the car on Sunday just to give it some fresh air and change the fuel in the carbs, went for the clutch and got a dangly pedal instead. After a couple of prods it got some pressure back, but not enough to disengage the clutch. No fluid leak anywhere. Anyway, had no time to investigate. Later on chatting to a chap in the local car bits shop (Karparts in Cainscross, Stroud, NTDWM, but bl**dy good) and his first question was "Silicone Fluid?" - funnily enough, yes, it is loaded with silicone. He reckoned there are problems encountered with said stuff causing seal sticking/hardening - anybody else able to shed some light/experience on this? Both Master (ex-Nissan) and Slave (original CBR1000) were stripped and cleaned before the fluid was added, and the hose is a new Goodridge item. I only went with silicone for the less corrosive nature of the stuff. Well ticked off.

Silicon fluid as in DOT 5?

I was under the impression that filling a system with DOT 5 that was designed for 3 or 4 (or 5.1 I guess) was a bad move and you'll end up wrecking all the seals if you do.

I am a fan of DOT 5 silicone fluid I use it with no problems. BUT I would not use it in a system which has previously had ordinary fluid in it, or a new system with second hand seals which had been previously exposed to ordinary fluid.

Cheers

Mark

New cyclinders/calipers are often lubricated with DOT 3 or 4 so if you fill with DOT 5 then you'll get problems. You need to make sure you get absolutely ALL of the old fluid out.

My understanding is that Dot is a specification relating to performance, i.e. hygroscopy, termal stability etc, not an indicator of its chemical composition - so Dot 5 can be silicone, mineral or whatever. I also understood that 3,4 or 5 are compatible if they are of the same fundamental components, i.e. mineral.
My gues from what has been written is that the seals had become impregnated over the years with traditional fluid that has now, after 2 years+ reacted with the seals to give the hardening. I'll get them out in the next couple of days and report back.

I think I've got this right but don't quote me.

Silicone fluid is not as hydroscopic as conventional fluid so any water tends to pool in the calipers, and boil under race conditions, potentially locking the brakes on.

I would have used Dot 5.1 fluid which is compatable with Dot 3&4 and has a much less corrosive nature. As you have started using silicon fluid you might have to change all the seals to convert back again so the choice is yours.

You must use new parts if using dot 5 as said above. It reacts with dot 3 and 4 and will cause problems like u have. Dot 5.1 can react with 3 and 4 aswell. If u wash things out its no good itl still be in the rubber seals.

Putting it nicely u will probably have to replace/rebuild your parts.

DOT5.1 is fully compatible with DOT3 and 4, but mixing them will reduce the specification of the DOT5.1 fluid, e.g. boiling point will be lowered.

A lot of misinfo out on the web about brake fluids some MG owners sites state side will tell you that they must have DOT 3 rather than DOT 4 for thier cars or the seals fail which is of course guff. Dosen't help that that the designation DOT 5 has different interpretations depending on which side the atlantic you are.



Dot 5 isn't a problem it is compatible with older systems and is the fluid of choice for cars in long term storage, but it gives a slightly spongeier pedal and is less good at lubricationg the seals and pistons. I personally wouldn't use it in an everyday day car but it is fine for 50s and 60s classics where it will prevent seized wheel cylinders if the car is stored long term.

DOT5 isn't hydroscopic at all unlike the conventional DOT5.1 As a rule of thumb with conventional fluids the higher the dry boiling point the more hydrocopic the fluid. Mixing silicone and conventional fluids isn't a good idea but won't make the wheels fall off as long as only a tiny ammount of "dry" conventional fluid was left in the system


As to what has happened in this case I suspect either one of the pistons has been has been sticking on the return stroke or the recurerating seal on the master cylinder is sticking open. Using old hydraulic parts without fully rebuilding them isn't a good idea.

quote:

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Originally posted by britishtrident
Dot 5 isn't a problem it is compatible with older systems

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There is one major problem that can give a soft pedal - it stores air in suspension very easily. If you pour it in too fast it takes in gas from the bubbles, leaving you with a spongy pedal that can take ages to fix. The other problem is that, being non-hygroscopic, it holds water in remote parts of the system rather than absorbing it into the fluid, to be replaced at a later date. This 'stored' water can either turn to steam when braking hard, or cause rust when in storage.

I used to have silicone fluid in my car's braking system, wasn't happy with it, and replaced it with good standard fluid. Unfortunately, it took a LOT of bleeding and pumping to get the fluid to run clear. On the plus side - my brake pedal is a lot more solid now! I decided to just accept the fact that I would have to replace the fluid every year or two.

As an aside, silicone fluid was developed for the US military many years ago, to make servicing less of a worry. They have now decided that it wasn't worth the fuss, so the manufacturers' major customer isn't buying it any more. Very soon it's going to be VERY hard to buy!

David

While Silicone fluid is supposed to last the lifetime of the brake/clutch sytem it does have a few issues.
Silicone brake fluids are not hygroscopic, and tend to retain their dry boiling points for very long periods of time. For this reason, silicones are favored by owners and restorers of classic and antique cars, as there is minimal danger that seldom-used and possibly irreplaceable brake components will be lost to corrosion.

Silicone will cause natural rubber to swell, even when it's compounded with synthetics. The seals in modern brake systems are no longer 100% natural rubber, but blends of natural rubber and synthetics like nitrile. Glycol fluids will also tend to swell blended rubber seals, but to a much smaller degree then silicone. Swollen seals may leak, or cause caliper pistons to bind.
Silicone has several other properties that make it less then desirable for street or track use. When forced thru small orifices under high pressure, it tends to foam, generating bubbles. Bubbles in fluid make for spongy pedal feel. Silicone also tends to become slightly compressible at temperatures near its boiling point, which makes it generally inappropriate for racing


I personally use ATE Super Racing Blue and have done for many years without issue. It has been used in most of our race cars and as long as you observe the std procedures of changing the fluid every now and again especially if you manage to boil it then you should he no problems.

[Edited on 7/6/06 by atomic]

I experienced similar problems with silicone fluid i.e. spongy (to the metal!!) pedal when worked v hard and decided to convert back.

Blew the system clear flushed and refilled. No problems - no spongy pedal - solid as a rock.

One month later noticed what looks like snott floating in fluid. Went through same process again.

Next month - you guessed it!

Now on 6th flush and replace all has been OK since May but I'm watching it.

As an aside I have noticed signs of pitting in the ally reservoirs - were brand new last year.

I will never use silicone again ever.