Brake Fluid

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Brake Fluid

Post by Tommy »

I have put this here because the only technical subject under 'technical discussion' is about engines.

Does it matter whether, when changing your brake fluid in a 541, C-V8 or Interceptor, you use Dot 3 or Dot 4 brake fluid?

I realise that Citroen cars and Rolls Royces use a totally different type of brake fluid but, for the normal systems, I have always assumed that it is better to use the latest version, i.e. a Dot 4 rather than Dot 3, but is this correct?

The reason I ask is that it has just been discovered that manufacturers of the new 'red' coloured Anti-Freeze have, without warning, put something in it that can attack hoses, seals and head gaskets of pre-1990 cars. (In my view they should be sued for this because they should carry a clear warning on the label - which they have only just started to do, but in miniscule type!)

In view of this, I wondered whether there could be a similar problem with using a too-new brake fluid??

Does anyone know the answer?
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Post by VFK44 »

I'm waiting for an expert to reply, but in the meantime I can add my penn'orth.

DOT3 and DOT4 are supposedly compatible and can theoretically be mixed. Far from DOT4 causing problems, I understand that it was DOT3 that caused problems with rubber seals in some older cars. (They can swell up - not sure if this is caused by "proper" DOT3 or just some manufacturers versions). They are Glycol based.

DOT5 is silicon based, does not absorb water but can contain water as droplets which will sink to the lowest level. Some folks think it encourages corrosion, others think it cures it. Driving performance is marginally less as it can be compressed more, but not so you would know it on the road - the bigger problem will be getting all the air out of the system. It should only be added to a completely stripped and cleaned system. It does not strip paint, rot plastic or catch fire. As ordinary fluid is more dangerous than petrol, this is a big big plus!

The complicated part is that the DOT ratings were intended as a scientific test of the fluid's performance, not what it is made of. So theoretically some DOT 3 fluids reached DOT4 performance, and DOT4 can exceed DOT 5! Presumably whisky could be given a DOT label if you wished to test it... This means that the latest Glycol fluids have been labelled as DOT5.1 - even though they are COMPLETELY incompatible with DOT5. They can supposedly be mixed with DOT3 & 4. What a daft system!
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Post by michael125 »

though I´m an automotive engineer my english is too poor to described it exactly, and I dont know the original specification of the brake fluid in a Jensen car.

you are right, Polyalkylenglycoläther DOT 3, DOT 4 and DOT 5.1 are kind of updates, but it is better to use the original old spezifikation (DOT 4 is more agressive to the rubber seals than 3 and so on)

DOT 5 is a completly different Silicon fluid that refuses water and let it stay in drops somewhere in the system, the main danger is not the corrosion or the waterdrop changing to gas at 100 Celsius (and pedal falling down) the main problem is if such a drop frezes somewhere in the brake pipe, than there will be no brake... nothing... whether the rubber seals like silicon is not the question, never use DOT 5 in a 3/4/5.1 System!

Some Citroen and Rolls Royce use mineral oil based hydraulik fluid for there braking and suspension system, this is the right fluid to destroy the braking system of any other car
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