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Brake fluid, and why 5.1 isn't the best.

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5.1
24K views 17 replies 7 participants last post by  mkstyle 
#1 ·
You might have noticed that there are a number of different brake fluid types available, all classified with a "Department of Transportation" number (DOT, this is the US D.O.T , the UK is Department of Transport.

There are 2 main classes, Silicone based,(DOT 5) and Glycol based. (DOT 3/4/5.1)

Glycol based fluids will pull water out of the air, and reduce their boiling point, Silicone based fluids won't, but using Silicone in a system that has had other fluids in it can lead to water and moisture pooling in areas, which can boil and ruin your brakes.

Glycol fluids mix with the water and hold it, it will reduce the effectiveness, but it'll be gone the next time it's changed.

You're unlikely to see anything these days with DOT5 in it, likewise for DOT3 which has been replaced by DOT4.

What can be mixed?
Do not mix DOT5 with anything, DOT3, DOT4 and DOT 5.1 can be mixed with each other without problems, but it'll be as bad as the weakest link, which may be old damp fluid, or a lower rated fluid added into a better system.

What are the differences?

DOT3 -
dry
205 °C
wet
140 °C


DOT4-
dry
230 °C
wet
155 °C


DOT 5.1
dry
270 °C
wet
190 °C

The important bit is these are the MINIMUM boiling points allowed to be in a certain spec.

What's the best fluid then?

If you are stuck, and have only a few options, then you'll probably find that a DOT 5.1 will be the best performing, but there are better fluids out there.


ATE Super Blue ~ £20/litre
dry
280 °C
wet
192 °C

Millers Racing Brake fluid ~ £14/500ml
dry
310 °C
wet
194 °C

Motul RBF600 ~£15/500ml
dry
312 °C
wet
216 °C

Motul RBF660 ~£20/500ml
dry
325 °C
wet
204 °C

Castrol SRF ~£40/litre
dry
310+ °C
wet
270 °C


All of the above fluids are in fact DOT4s, and Castrol SRF is DOT 3/4. Castrol SRF doesn't show a boiling point in most documentation, just a statement that it's easily over 300°C, and it's Wet boiling point is massive compared with most others. It will soak up water far quicker than any other the other types, and usually is used for racing applications where changes are far more frequent.

It just goes to show that all of the best fluids are DOT4 rated, so be aware that getting a good DOT4 is always better than an average DOT 5.1
You won't feel a difference between each type, that's all down to hydraulics, but for track work, boiling the fluid will ruin your day, and potentially be quite dangerous.
 
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#3 ·
I've tended to use Super blue, as it has a lovely colour (it is actually blue, so you can see it bleeding though to the calipers) and it's easy to get. Either Eurocarparts or GSF sell it. I forget which one, but it means you can often pop out and get something decent on a saturday.

The rest are all available online in various places, Opie Oils stocks all of them apart from super blue, and a lot of them are better still, but usually more expensive. That's why i included price / size for a better comparison.

I haven't had any problems with superblue as yet, but as you can see, there are even better ones out there.
Prices are a rough guide, so shop around if you are buying on the 'net.
 
#4 ·
Hi,

Will be bleeding through my brakes next weekend as i bought some braided hosing.

Trying to decide between Castrol SRF or RBF 600

SRF is very pricey, i can't get it trade :frown so have to pay full RRP can get the RBF cheap though :hihi

Is SRF worth it ? I don't do track events (yet) but do subject the brakes to some heavy applications....
 
#8 ·
Is RBF 600 going to be ok in a 98 E39 523 ? I've no idea what the stock fluid is in them :frown

How much fluid am i going to need ? It comes in bottles of 500ml. I will be removing my calipers and fitted new hoses, hope to totally drain all the oil fluid out the system.
 
#10 ·
Hi,

What i meant was in terms of it's type rather than its performance, im not sure what DOT is used in the car from stock (im guessing it's probably still stock!)

I'm having to mail order it as i can't find any suppliers round here, don't want to not have enough will 500ml defo be enough ?
 
#12 ·
Is there any need to spend £££ on fluid if your car is for normal road use only? Would i feel any difference? As i can get Vauxhall/GM DOT 4 from work for free! But if there is any difference in braking feel with more advanced fluids i may be tempted.......
 
#13 ·
For the most of it, you'll be well within the limits of regular stuff. It's rare to get the brakes to the temps to boil dry fluid on the road unless you're a bit of a part time rally driver.
Wet/damp fluid is a different story though, so as long as you're driving sensibly and changing your fluid when needed you'll be fine.

I would say get some good stuff if you are towing a caravan, heading down a long hill dragging the brakes will overstress most fluids.
 
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