M cars brake fluid?

[Dakkon]

Does anyone know what fluid if used as standard on an M car if a BMW main dealer services the car? For a lot of cars it is dot 4, is this the same for a Z4M for example?

[jeffm3]

change it for castrol SRF the only choice! double the price but the fluid doesn't boil so you won't loose your brake pedal great stuff! dot 4 ok for standard bmw but if your using a performance bimmer go for srf

[mmm-five]

jeffm3 said:

change it for castrol SRF the only choice! double the price but the fluid doesn't boil so you won't loose your brake pedal great stuff! dot 4 ok for standard bmw but if your using a performance bimmer go for srf

SRF (DOT5) is overkill if it's only for road use - normal DOT 4 or DOT5.1 will work fine. DOT5.1 works at the same temp range as DOT5 SRF.

You'll also need to regularly change SRF as it 'goes off' and is less amenable to water contamination (i.e. DOT4/5.1 fluid absorbs water in the system, reducing it's effectiveness gradually, whereas DOT5 fluid doesn't absorb water so you end up with parts of the braking system that are water and not fluid) and is more c

[jeffm3]

mmm-five said:

jeffm3 said:

change it for castrol SRF the only choice! double the price but the fluid doesn't boil so you won't loose your brake pedal great stuff! dot 4 ok for standard bmw but if your using a performance bimmer go for srf

SRF (DOT5) is overkill if it's only for road use - normal DOT 4 or DOT5.1 will work fine. DOT5.1 works at the same temp range as DOT5 SRF.

You'll also need to regularly change SRF as it 'goes off' and is less amenable to water contamination (i.e. DOT4/5.1 fluid absorbs water in the system, reducing it's effectiveness gradually, whereas DOT5 fluid doesn't absorb water so you end up with parts of the braking system that are water and not fluid) and is more c

Agree if just road use, but it makes a difference after a few laps! don't understand the end of your post "and is more c" ??? must of submited without you finnishing word i'm guessing!

Edited by jeffm3 on Friday 4th July 07:07

[Huliganov]

You only need Dot 5 if you do lots of track days or drive a big heavy vehicle like a truck, large van etc.

Dot 4 is more than suffecient for your needs.

[Mroad]

There is a lot of misinformation in this thread and I have seen it before in other postings.
The trouble comes when people start quoting DOT ratings with sweeping statements like a DOT 5 or DOT 5.1 fluid is better than a DOT 4 fluid, well I'm afraid you can't make statements like that because in most cases it isn't true.

Firstly to set a few things straight.

Castrol SRF is DOT 4 rated and is definitely not DOT 5.
DOT 5 rating is reserved SOLELY for Silicone based brake fluids and these should not be used in performance applications. Basically any moisture entering the system will stay in small bubbles or pockets as water cannot be absorbed into silicone fluids. As soon as the fluid reaches around 100C (pressure dependent) the trapped water will boil. As gases can be compressed and fluids can't your brake pedal will become softer. Separated water in the system can also lead to corrosion on metal components (pistons etc.). Silicone fluids should not be mixed with other DOT rated fluids as they are completely different chemically.

DOT 3, DOT 4 and DOT 5.1 brake fluids are Glycol-ether based fluids. Glycol-ether is hygroscopic which means it absorbs water into the fluid (water combines with the fluid, water doesn't sit separately as with Silicone fluids). As water is absorbed into the fluid it will gradually reduce it's overall boiling point. For this reason fluids are rated with with two boiling points, one with no water absorbed into the fluid (Dry Boiling Point) and one with 3.5% water absorbed into the fluid (Wet Boiling Point).
The Wet and Dry boiling points are the MOST important factor when deciding what brake fluid to use, DOT ratings set the minimum requirements for these two boiling points. The higher the DOT rating, the higher the minimum boiling point requirement.
HOWEVER that is not the whole story as the DOT requirements also stipulate other criteria that the fluids must pass including acidity, corrosion protection etc. but the most important one for us after boiling points is viscosity.
With the introduction of high tech ABS/TCS/ESP systems there was a need for thinner brake fluids so they could pass easily through the micro valves used in these systems. The main reason why the DOT 5.1 rating was introduced was to set a standard for these thinner fluids.
DOT 5.1 criteria dictates that the fluid will be thinner in viscosity than a DOT 4 fluid.
If your manufacturer says you only need a DOT 4 rated brake fluid (as mentioned above, all BMW's only need DOT 4) then you don't need a DOT 5.1 fluid. In fact there aren't many cars that specifically require a DOT 5.1 fluid.

If you check the Wet and Dry boiling points of the most popular DOT 4 and 5.1 fluids you will find the best performing fluids (like Castrol SRF) are in fact DOT 4 rated. The reason they are DOT 4 rated is they are too thick to pass the DOT 5.1 viscosity criteria even though they surpass the DOT 5.1 wet and dry boiling point criteria by miles!

A quick check with manufacturers shows...

Brake Fluid	DOT rating	Dry boiling point min. °C	Wet boiling point min. °C
Castrol SRF	DOT 4	310°C	270°C
Motul RBF 600	DOT 4	312°C	216°C
Motul RBF 660	DOT 4	325°C	204°C
AP 600	DOT 4	300°C	204°C
ATE 'Super Blue'	DOT 4	280°C	200°C
AP PRF 660	DOT 4	320°C	199°C
Castrol Response Super DOT 4	DOT 4	280°C	190°C
Motul DOT 5.1	DOT 5.1	275°C	185°C
AP Formula DOT 5.1	DOT 5.1	275°C	184°C
Castrol Response DOT 4	DOT 4	265°C	170°C
AP 551	DOT 4	275°C	150°C

The really important numbers (to me anyway) are the Wet boiling points, SRF wins hands down with this and you can see why it is so widely regraded as the ultimate brake fluid. What the wet boiling points don't tell you however is how quickly the brake fluids can absorb moisture, some fluids are more hygroscopic than others and will get to the wet boiling point quicker than others. Manufacturers can get around this by treating the fluid with Boric acid to help neutralize any water entering the fluid and makes the fluid slower to drop to the wet boiling point. Castrol use this method with their 'Super Dot 4' giving it a longer life. ATE 'Super Blue' is also a very good performer over a long period and I believe was the OEM fluid for BMW's for a while (I have used it in the past and it's been excellent).
Whatever fluid you chose change it AT LEAST every two years and preferably annually.

Personally I use Motul RBF 600, it's half the price of Castrol SRF with most of the benefits.

Edited by Mroad on Friday 4th July 15:52

Edited by Mroad on Friday 4th July 15:53

[jeffm3]

That puts a end to this thread lol, great post you obviously know your stuff, i have used SRF on recomendation not coz i know the specifics and it does performs better than anything else i've used!

[Chessers]

What a great post - I use ATE Blue + stainless hoses and it made a real difference to my car.

[Mroad]

Sorry for the long post, I'm no expert, I just looked into brake fluids about 10 years ago while trying to find the best one for track use. It can be confusing and the regulations don't help, I mean fancy having DOT 4 as Glycol, DOT 5 as Silicone and then DOT 5.1 back to Glycol? No wonder people get it wrong.

jeffm3 said:

That puts a end to this thread lol, great post you obviously know your stuff, i have used SRF on recomendation not coz i know the specifics and it does performs better than anything else i've used!

SRF is the definitely the best fluid but I have heard absorbs moisture quite quickly (not sure how true that is though but I've heard it from a few places). Obviously that's not a problem if you are a race team changing the fluid every race but could be for road use, changing it every 2 years (BMW recommendations). The specs of SRF are so high in the first place though that it is unlikely to be a problem and I know of a few people that use it with absolutely no problems (but like me, they change their brake fluid every year).
I would use it but I don't want to pay £40/litre

Chessers said:

What a great post - I use ATE Blue + stainless hoses and it made a real difference to my car.

I used to use Super Blue, a good fluid that definitely should last 2 years of everyday use. I used it on track too without a problem before switching to Motul RBF (bit cheaper, better specs and ATE was getting hard to get hold of). Incidentally in case you didn't know, ATE Super Blue is well...blue BUT you can also get ATE TYP 200 which is exactly the same except it's amber in colour. It makes changing the fluid easy if you switch between the two, just bleed it until you see the other colour fluid come through.


A few general tips about brake fluid...

Only buy enough fluid for what you need at the time. Fluid sitting on shelves just absorbs moisture from the atmosphere so when you come to use it, it maybe no good. Incidentally that is why decent fluids come in metal cans (ATE for example and SRF used too), moisture cannot pass through metal where as it can permeate through plastic.

Speaking of moisture permeating through plastic, tests have proven that the most moisture entering a brake system comes from...moisture permeating directly through the rubber of the brake hoses! Remember that the next time you drive through a puddle

Edited by Mroad on Saturday 5th July 13:42

[jeffm3]

I change the fluid everytime i change the pads which is about once a year if i have the car for that long, and if i buy a new car i'll change them and fluid whatever they look like (piece of mind) i change it every year and pads regardless of milage but there is usually plenty of mls done on them, coz IMO your brakes can not be neglected and pads left until they are low i will change mine probably well before most people, i had a mate that thought it was really funny to have the metal backing of his brake pads rubbing on the discs (down to the metal) ok st happens and maybe didn't have the money to replace them, well don't drive it then? stupid don't come close to what i called him, wouldn't be too happy if he run into the back of me coz his brakes were sh17 and hurt one of my kids, so its nice to have good brakes for when i'm on the track but i do what i can for a safety reason more than anything (three kids in the car)

Anyway got a question for you with this water larky i have metal hoses would having metal ones make a difference then yeah? and if i change the fluid once a year using SRF i'll be o.k as i don't want to use it if it perishes after a 6 month and doesn't perform as good.

thanks in advance jeff!

[Chessers]

Having braided hoses does make a massive difference - well it did on mine anyway.

You get a much better feel on the brake pedal as the lines don't expand like rubber ones. Not sure if your fluid lasts any longer before it starts breaking down.

[jeffm3]

Chessers said:

Having braided hoses does make a massive difference - well it did on mine anyway.

You get a much better feel on the brake pedal as the lines don't expand like rubber ones. Not sure if your fluid lasts any longer before it starts breaking down.

It made a difference to mine too, i was more conserned with this water ingress happening or not? does anyone know if it stops this happening having metal hoses, i'm guessing it does but would be nice to know for sure

Edited by jeffm3 on Tuesday 8th July 10:57

[Mroad]

Braided hoses probably won't make any difference to the water ingress, it is only a braid over the top of rubber or PTFE hoses to stop them expanding under pressure and gives a firmer pedal. They are a good and cheap upgrade, I have put them on all my performance cars.
Don't worry about the water ingress, it is minimal. I was just pointing out that is where the majority of moisture enters the braking system, most people logically think it's through the cap or around the piston seals. If you are changing the fluid every year then you are doing a good job (a lot better than most) and have nothing to worry about.

[smack]

Where is the best place / price to the the ATE or Motul fluid?

Super Blue seems to be 16.50/1L RRP, and if it is in a Silver tin it is old stuff (should be blue tin) according to one retailer. On fleabay it under 10 for a silver tin.

RBF600 looks like it is just under 10/1L RRP.

When I worked in motorsport, we ran Castrol SRF in the car. But one of our sponsor/suppliers was Mobil - so we had to decant the SRF into a Mobil tin... It also went into disk brake systems used on our own mountain bikes

Might have to give them a call to see if a tin can get lost for me..

[Mroad]

RBF600 £10/litre? BUY IT! Think you mean £10/500ml or £20/litre? I got my last lot from Opie Oils, think it was around £13/500ml.
ATE comes in a 1 litre tin unless they have changed it, £16.50/litre sounds about right.

I don't know what colour tin ATE Super Blue is now, when I was buying it about 7 or 8 years ago it was in a blue tin. ATE TYP200 comes in silver tin I think, exactly the same stuff as Super Blue except it's amber in colour.
I remember trying to get some Super Blue from a BMW dealership just up the road when I was desperate for some (track day next day, bleeding the brakes and ran out) and they got totally confused when I asked for it until they found it listed as ATE "RACING" Super Blue
I have no idea if it's still the BMW OEM fluid though, probably not.

That'll be some serious braking to warrant using SRF on your mountain bike!

Edited by Mroad on Tuesday 8th July 22:59

[Mroad]

Thought I better look for ATE stuff. The old silver tin you mention was the one I used to get (silvery blue hence the confusion), seems they have changed the design to a completely blue tin.
Always get the freshest stuff you can so I would steer clear of the older stuff even if means paying a few quid more.

[jeffm3]

Mroad said:

Braided hoses probably won't make any difference to the water ingress, it is only a braid over the top of rubber or PTFE hoses to stop them expanding under pressure and gives a firmer pedal. They are a good and cheap upgrade, I have put them on all my performance cars.
Don't worry about the water ingress, it is minimal. I was just pointing out that is where the majority of moisture enters the braking system, most people logically think it's through the cap or around the piston seals. If you are changing the fluid every year then you are doing a good job (a lot better than most) and have nothing to worry about.

was thinking they are rubber inside but it might help too keep some of it off but yeah i get your point and thanks for the info, i'm sure as long as i keep doing every year it will be fine, just to say aswell when i do change the pads and fluid even just after a year and even if pads aren't very low you don't half notice a difference, i think because the performance loss is gradual over time you don't notice just how poor they are compared to when you do the work, just my opinion so well worth changing it even on a non performance car?
cheers mroad

[smack]

Mroad said:

RBF600 £10/litre? BUY IT! Think you mean £10/500ml or £20/litre? I got my last lot from Opie Oils, think it was around £13/500ml.
ATE comes in a 1 litre tin unless they have changed it, £16.50/litre sounds about right.

Ah yes, you are right, it was for 500ml. But it would be interesting to see how much it is in the trade - I'll ask the purchasing guys at my old work. If they can get it for a good price, I could buy a box of fluild if anyone else is interested?

Mroad said:

I don't know what colour tin ATE Super Blue is now, when I was buying it about 7 or 8 years ago it was in a blue tin. ATE TYP200 comes in silver tin I think, exactly the same stuff as Super Blue except it's amber in colour.
I remember trying to get some Super Blue from a BMW dealership just up the road when I was desperate for some (track day next day, bleeding the brakes and ran out) and they got totally confused when I asked for it until they found it listed as ATE "RACING" Super Blue
I have no idea if it's still the BMW OEM fluid though, probably not.

That'll be some serious braking to warrant using SRF on your mountain bike!

We take our bikes to the Alps, so they do get a little hot
But they main reason was "Because we can"!

[puntograle]

Does anyone have any experience of how often Dot 4 brake fluid will need to be changed given its water absorbtion qualities? I was thinking of the Motul RBF 600 for an E39 M5. One supplier suggested it would need changing every 6 months....does that sound right?
I was thinking of changing it every year....at worst, would every two years be unrecommended?

Thanks.

[Mroad]

jeffm3 said:

Mroad said:

Braided hoses probably won't make any difference to the water ingress, it is only a braid over the top of rubber or PTFE hoses to stop them expanding under pressure and gives a firmer pedal. They are a good and cheap upgrade, I have put them on all my performance cars.
Don't worry about the water ingress, it is minimal. I was just pointing out that is where the majority of moisture enters the braking system, most people logically think it's through the cap or around the piston seals. If you are changing the fluid every year then you are doing a good job (a lot better than most) and have nothing to worry about.

was thinking they are rubber inside but it might help too keep some of it off but yeah i get your point and thanks for the info, i'm sure as long as i keep doing every year it will be fine, just to say aswell when i do change the pads and fluid even just after a year and even if pads aren't very low you don't half notice a difference, i think because the performance loss is gradual over time you don't notice just how poor they are compared to when you do the work, just my opinion so well worth changing it even on a non performance car?
cheers mroad

Changing the pads every year is over kill unless the wear warrants it. It's obviously better to change them sooner than leave it too late. The difference you notice with new pads over semi worn ones is the pedal travel will be slightly less. With the new pads in and the pistons right back in the caliper you don't have to move as much fluid to get the same pressure on the pistons/pads.