overdue brake fluid change - DOT5

[CPGCharlie]

Hi All, 

I need to change my brake fluid and the notes from the previous owner and builder were to use DOT5 on AP racing 4 pot on the front and standard Caterham on the back. Finding DOT 5 has been difficult and also the colour of the brake fluid at the moment is yellow... I believe DOT 5 is purple and shouldn't be mixed... 

Do I need DOT 5? if not what grade should I use instead? 

many thanks,

Charlie 

[simon_h]

I’d just use normal dot 4 and change it occasionally 

[Fyldeflier]

Hi Charlie.

I think you need to check if you have dot 5 or dot 5.1

dot 5 is silicon and you must not mix it with other types of brake fluid.

dot 5 will not attract moisture and therefore you shouldn't need to change it. When it came out it was touted as being great for racing due to its higher boiling point. However it is somewhat more compressible than normal brake fluid which gives a longer or spongier feel to the brake pedal so it fell out of favour.

when it is fresh it is often purple but with time it changes colour and eventually ens up yellow ish or clear.

Hope this helps.

 

John.

 

[ECR]

I lifted this from blatchat years ago and can't remember the OP...or vouch for accuracy but its interesting nevertheless ...

Brake Fluid Details

So these are the "standard" specs for the various DOT (Department Of Transport) specs.

              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    260 °C (500 °F)    180 °C (356 °F)DOT 3, 4 and 5.1 are all polyglycol ether based fluids and cannot be mixed with DOT 5.0, which is silicone based.

Interestingly some dot 3 fluid have a dry boiling point of 275 degrees! (AP 551 Racing Brake Fluid which is what I use)

So whilst DOT 5.1 looks (is) better in terms of its dry boiling point I would suggest that unless your car is only ever used on track (as mine is) and you have very aggressive friction material (which I do) you will never need to go beyond DOT4! That is because the frictional rotational retardation is turned into heat at higher levels. (jumping on the brakes at the last moment)

Castrol SFR is similar to DOT3/4 but with a dry boiling point of 300 c 

SFR typically costs £60 for 1 litre, whereas the AP 551 costs about £17.50 per litre

[Geoff Brown]

If one does not want to seek out DOT 5 & pay the price then opt for the recommended common or garden DOT 4 but it will require the brake system to be adequately flushed through. 

If DOT 4 is going to be the option  I would suggest flushing through with at least a litre. Then followed by another litre just to make sure.

Recommended change interval for Sevens on DOT 4 is every two years.
 

 

[7 wonders of the world]

DOT 4  ATE typ 200 has a dry boiling point of 280 degrees and is readily available for a meager price.

[bluenose]

I have used ATE super blue in the past and changed to ATE type200 amber but the super blue is not for road use because of the dye so does not meet the dot4 spec.  

 

[ScottR400D]

I doubt that simply flushing through will be good enough. There's a lot of stuff on the web about what happens when DOT 5 mixes with 3, 4, 5.1.

Seal damage is one issue and the problem of water based corrosion with DOT 5 is fairly well known. 

I wouldn't be happy with less than a full strip down to make sure ALL the DOT 5 is gone, there's no corrosion and to replace all the seals. 

That is if it actually is DOT 5 and not 5.1. 5 is generally considered to be lower performance than 4/5.1 so it's hard to see why anyone would use it in recent years. 

[aerobod - near CYYC]

In the unlikely event that silicone DOT 5 has been used in the braking system, there is a simple test to do, courtesy of Ferrari Chat (https://www.ferrarichat.com/forum/threads/is-there-a-definitive-way-to-tell-if-you-have-silicone-brake-fluid-in-your-car.299096/):

"Take some of the fluid out of the master cylinder place it in a container then add some DOT-4 to the same container. Shake the container to mix them up and then let it sit and see if the 2 separate. DOT-4 and DOT-5 will separate as they are not compatible.

 

If you have DOT-5 in your system and you want to switch to DOT-4 you will need to rebuild the entire system replacing all of the rubber components."

[ScottR400D]

Excellent tip! 

[CPGCharlie]

brilliant, many thanks all. 

[Geoff Brown]

The mantra is then - avoid DOT 5 at all costs.!