I just fitted the DS 2500 along with Zimmermann drilled discs. Altough the pedal travel increased, braking highly improved, especially at high speeds !

Not sure how boiling water leads to air bubbles? :confused:

Ever watched your kettle boil?

wonder where the bubbles come from..

Ever watched your kettle boil?

wonder where the bubbles come from..

Couldnt have put it better myself

Ever watched your kettle boil?

wonder where the bubbles come from..

Is that how they get the bubbles in lager?

If so does that explain why lager is crap as brake fluid.

I regularly used to boil the fluid in my mountain bike brakes.

DOT 5.1 is compatitable with DOT 4.

However if you mix the fluids you will alter the properties of DOT 5.1 bringing the boiling point down.

Ever watched your kettle boil?

wonder where the bubbles come from..

The bubbles are the more energetic molecules escaping as gas, if the brake fluid is boiling and its a single component (doubt it) then the bubbles are gaseous brake fluid (or a gas of the more volatile components).

As the fluid boils the system will pressurise if it is sealed. If it remains sealed then it will continue to pressurise but then contract again as it cools and the gas condenses. If it releases pressure when hot and then cools it will either draw a vacuum, or more likely, pull in air which contains water.

I knew that my education would come in useful one day

Stephen

Yup - so we're all in agreement - the bubbles can't be air, but I can see how air might be drawn in, especially if the fluid level in the reservoir was low (must check). However I would have thought that more likely the residuum would just be replaced by fluid from the reservoir - which I guess would be more likely to contain water, being at the highest point in the system and being (partially) open to the atmosphere. However, at normal temperatures I'd expect water to be not much less compressible than brake fluid, so it still remains somewhat of a mystery how the pedal can be so much softer after the system has stabilised again... :rubchin:

it does'nt have to be air to be a bubble... (other gas equally compressable)

you get bubbles in vacuum.. it aint air.. they're called voids then..

confusing is'nt it..

As the fluid boils the system will pressurise if it is sealed.

It isnt sealed, its vented to atmostphere.

All fluids have a boiling point, some DOT 4 fluids have a higher boiling point than others, these are usually branded Super DOT 4's. DOT 5.1 is one step up from DOT 4, but if you get a lesser DOT 5.1 its boiling point can be lower than some Super DOT4's.

Confused yet?

it does'nt have to be air to be a bubble... (other gas equally compressable)

you get bubbles in vacuum.. it aint air.. they're called voids then..

confusing is'nt it..

Bill' date=' I'm not confused. I KNOW it isn't air. Hence my remark - "Not sure how boiling WATER causes AIR bubbles" (without the capitals, originally). All I'm wondering is how in a closed system, the boiling of the fluid (whatever mixture it happens to be) can bring about a [i']permanent[/i] change in the compressibility of the said fluid. Make sense?

The system isnt closed.

The system isnt closed.

Nope, I know that too. There's a vent under the reservoir cap. This implies that you're suggesting that when the fluid boils, it then sucks air into solution from the top surface of the fluid in the reservoir, right down past the ABS actuator to the pipe at the wheel end? Because surely air trapped above that point is just going to work its way back out again, back to the highest point.

I'm not being awkward for the sake of it. I'm going to change my fluid anyway. I shall watch with interest for any air bubbles. However, I'm interested in the physics of the thing. I have an engineering degree, and I've been tinkering with cars since I was old enough to hold spanners for my Dad, or press the brake pedal for him while he bled brakes, so I know roughly how cars work. I can visualise bubbles forming in a fluid when it boils. I can visualise air being absorbed from the fluid surface in the reservoir (although it's a pretty small surface), and brake fluid has fairly high surface tension. But I'm struggling to believe that it's air sucked DOWNwards through the fluid.

I know - I'm an obstinate old git bear with me

This is quite good as a discussion though dont you think, better than ive got a black Fabia and its better than your yellow one...lol.

Remember the air/gas/water whatever doenst need to get down to the caliper to cause a problem, anywhere after the master cylinder will cause a spongy pedal. Get it in the ABS block and that when it really gets trapped and you need to operate the ABS pump to get it out.

Although brake fluid absorbs/attracts moisture over time, through cap breather, rubber lines etc this isnt going to be Colins problem as the fluid was replaced only a short time ago. A dead easy way to check if its a brake material fault of a fluid fault is to clamp off each caliper then try the pedal. If its still spongy then it can only be fluid, release the clamps one by one and you can even narrow it down to a corner if its a pads gone soft.

And rememeber its not an arguement, its a constructive discussion, a good learning tool for everyone.

http://auto.howstuffworks.com/question451.htm

Agreed, Ross. I love discussion. But we seem to have stopped dead. Colin and I have the same problem, from the same trackday, with the same pads, and the same fluid. I don't know when Colin changed his fluid (Col?) but mine was done just before Brunters. I had no brake fade then, and the brakes have been absolutely fine since. I wouldn't really call what I have now fade. The pedal is a bit softer, and seems to travel further before acting, but if I stand on the brakes, the car still stops pretty smartly. Pumping the pedal doesn't make it any firmer, either. If it was air / water ingested over time, my problem (or Colin's) wouldn't have suddenly appeared after Sunday. Hence we need another explanation

The pedal is a bit softer, and seems to travel further before acting

Could this be that you have less pad/disc left so you have to press the pedel further for the pad to meet the disc?

I've done over 10,000 miles on my DS2500's + standard discs & fluid now, and the brakes are as good as ever. Still plenty of meat on the pads too.

This set-up was totally reliable at Anglesey, and i did brake VERY late on a few occasions, one time at the hairpin springs to mind

I also found myself carrying a bit too much speed into / through School on a few occasions, and the brakes scrubbed off the excess speed quite nicely.

New fluid is due in March anyway, so i'll upgrade then

What you get is water in the fluid from the condensation. This is why brake fluids quote 2 boiling temperatures, wet and dry. The dry temp is when the fluid is new. The difference between a top spec Super DOT4 and a good 5.1 is about 5c in favour of the DOT5.1, the difference when "wet" is the other way round by a larger degree (think mine is about 35c higher than the 5.1). The other source of "water" in the fluid is in a non closed sytem is general atmospheric degredation. The higher spec ones are more hydroscopic (sp) and will degrade quicker - I think with the 5.1 you need to change about every 20k miles or less, Super DOT4 "should" last 30-40k and standard 4 about 40-50 (which is why they say change it at 40k). Harder use (eg trackdays) will accelerate this degradation.

Didn't think DOT 5.1 absorbs water?

"DOT3, DOT4 and DOT5. DOT3 and DOT4 are glycol-based fluids, and DOT5 is silicon-based. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't."

Oh... and ive just heard you can now get DS2500 for Brembo brakes..... stoptastic :eek:

Isn't the glycol chemically altered after boiling?

Will it's density change too after boiling making it more compressionable?

Didn't think DOT 5.1 absorbs water?

"DOT3' date=' DOT4 and DOT5. DOT3 and DOT4 are glycol-based fluids, and DOT5 is silicon-based. The main difference is that DOT3 and DOT4 absorb water, while DOT5 doesn't."[/quote']

True but DOT 5.1 is also Gycol based that is why it can be mixed with 3 and 4.

DOT 5 is a completely different ball game and usually needs different seals in the braking system iirc and needs a full system purge, competition only really.

This is quite good as a discussion though dont you think' date=' better than ive got a black Fabia and its better than your yellow one...lol.

[/quote']

What you trying to say! :P lol

Gonna have to change my fluid when I uprate the discs/pads as the pedal is now getting rather soft! :eek: Will probably go for Super DOT4 even though I can get DOT5.1 free.

Im trying to say that my black one is better than your yellow one so nerr.

I did change Colin's fluid when i fitted his 312's but it was far from the full system bleed ive have since done for the likes of TFboy, Faboka and Walkie, now have the propper electric bleeding equipment and would like to have another go at Colin's with either Super DOT 4 or 5.1 whatever he prefers, as I understand he needs new front discs maybe it could be tied in together.

Im trying to say that my black one is better than your yellow one so nerr.

I did change Colin's fluid when i fitted his 312's but it was far from the full system bleed ive have since done for the likes of TFboy' date=' Faboka and Walkie, now have the propper electric bleeding equipment and would like to have another go at Colin's with either Super DOT 4 or 5.1 whatever he prefers, as I understand he needs new front discs maybe it could be tied in together.[/quote']

nice one Ross