I see under technical matters in the NA/NB FAQ section it states that silicon brake fluid cannot be used.
I do not understand why this should be, particularly if the rating of the fluid exceeds the original requirements.
Is there a known incompatability issue material wise perhaps, if so this would be unusual?
My 1997 Mk1 MX5 (not Eunos -if relevant) is a base model - road use only, with no ABS so there wouldn’t be any issues in respect of viscosity affecting braking pressure sensing units.
I have successfully used silicon brake fluid in older classic cars (Sunbeam Rapier Fastback and Morris Minor Traveller) so am a bit perplexed.![Undecided]( "Undecided")
Steve V ![Smile]( "Smile")
I’ve used Automec silicone brake fluid in my previous cars. Fiat cinquecento sporting and Ford Fiesta zetec without problems.
My Dad has been using it in his Fiat Cinquecento sporting for the last 16 years and 178’000 miles later no problems.
I am tempted to use it in my mx5 too.
You have to pretty much rebuild the system first to get all the glycol fluid out of it.
Silicone fluid is more compressible and gives a spongy pedal.
Water isn’t absorbed and removed at the next change, but sinks to the lowest point and rots out the brake pipes.
What’s not to like? ![Smile]( "Smile")
When I’ve used silicon brake fluid in the past, I’ve fitted new cupro nickel brake pipes as a matter of course.
I think the spongy pedal issue is over stated and in my experience have not noticed much difference -if any.
Water shouldn’t be an issue in a well maintained system.
![Smile]( "Smile")
From Opie oils, see the bit at the end
brake fluids I decided to contact Millers Oils up in West Yorkshire.
Their Technical Director, Martyn Mann was on hand to give us some useful info… below is
Martyn’s article on brake fluids.
There is a degree of confusion regarding the specification of brake fluid and this article sets
out to clarify the situation.
The Department of Transportation (DOT) classifies brake fluids to defined specifications.
These specifications relate to their boiling points and chemical composition, both of which
are important. All currently available brake fluids are covered by one of the following
specifications; DOT3, DOT4, DOT5 and DOT5.1.
The laws of thermo-dynamics dictate that the energy from motion is turned into heat through
friction. A braking system only works efficiently if the fluid remains incompressible. If the
brake fluid boils, it turns to gas, which is compressible and the braking system becomes
“spongy” or in extreme cases fails completely.
A brake system is not perfectly sealed and moisture can get into the system and be
absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the
efficiency of the braking system, as described above.
The DOT specifies two reference tests for brake fluids.
* Dry boiling point - the boiling point of fresh fluid
* Wet boiling point –the boiling point once the fluid has absorbed moisture
(representing brake fluid after time spent in a real situation).
There are two main types of brake fluids:
* DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol
compounds.
* DOT 5, which are based on Silicone.
Note the two types of fluid are not compatible and must not be mixed in
a braking system.
SILICONE BRAKE FLUID (DOT 5)
Silicone based DOT 5 was originally introduced to give higher temperature performance over
glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it
does not absorb water.
However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG
fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy
[quote=Geoff Walton]
From Opie oils, see the bit at the end
brake fluids I decided to contact Millers Oils up in West Yorkshire.
Their Technical Director, Martyn Mann was on hand to give us some useful info… below is
Martyn’s article on brake fluids.
There is a degree of confusion regarding the specification of brake fluid and this article sets
out to clarify the situation.
The Department of Transportation (DOT) classifies brake fluids to defined specifications.
These specifications relate to their boiling points and chemical composition, both of which
are important. All currently available brake fluids are covered by one of the following
specifications; DOT3, DOT4, DOT5 and DOT5.1.
The laws of thermo-dynamics dictate that the energy from motion is turned into heat through
friction. A braking system only works efficiently if the fluid remains incompressible. If the
brake fluid boils, it turns to gas, which is compressible and the braking system becomes
“spongy” or in extreme cases fails completely.
A brake system is not perfectly sealed and moisture can get into the system and be
absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the
efficiency of the braking system, as described above.
The DOT specifies two reference tests for brake fluids.
* Dry boiling point - the boiling point of fresh fluid
* Wet boiling point –the boiling point once the fluid has absorbed moisture
(representing brake fluid after time spent in a real situation).
There are two main types of brake fluids:
* DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol
compounds.
* DOT 5, which are based on Silicone.
Note the two types of fluid are not compatible and must not be mixed in
a braking system.
SILICONE BRAKE FLUID (DOT 5)
Silicone based DOT 5 was originally introduced to give higher temperature performance over
glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it
does not absorb water.
However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG
fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy
The last two lines say it all. Silicone fluid plays havoc with ABS pumps and piston seals. They tend to have a short life.
Regards Geoff Peace.
Not absorbing water is actually a bad thing for brake fluid. There are two things going on here, so I’ll explain.
Normal glycol fluid is hygroscopic. This is bad because it actively absorbs water from the atmosphere (mainly through the vented master cylinder reservoir). Score one to silicone. However, the water dissolves completely in the glycol and will do no harm (rust or reduced boiling point) provided its concentration doesn’t become too great. Hence the two-yearly fluid change, or annually if you can be bothered.
With silicone water will still get in, either by osmosis through flexible brake hoses or just due to the heating and cooling cycle under the bonnet (park the car with a hot engine and the master cylinder will draw in moist night air as it cools down). Because the water cannot dissolve in the silicone, it runs to the lowest point, generally a brake pipe somewhere. There it stays, because no-one ever changes silicone fluid. Water won’t rot a copper brake pipe but if it finds its way to a brake cylinder it can still boil, with the usual distressing consequences.