A question about brake fluid.

[Loz]

What exactly happens to brake fluid when it gets too hot, apart from boiling that is, & why does it need replacing?

 

[herb]

What exactly happens to brake fluid when it gets too hot, apart from boiling that is, & why does it need replacing?

have you been racing round the wolvo track again:amuse:

 

[obi_waynne]

Brake fluid is Hygroscopic (it attracts water) as it gets older it gets more and more saturated with water (and as we know water boils at 100 degrees which is fine for making a cup of tea but in a car the brakes will go spongy under heavy braking as the temp rises.)

Effectively the boiling temperature of the bake fluid reduces the more moisture it attracts.

Brake quality is measured with a brake fluid safety meter which most garages have.

I believe that dot 5.1 fluid is not subject to this problem althought it does cost a bit more.

 

[Loz]

have you been racing round the wolvo track again:amuse:

LOL! No, but it seems I had boiled the fluid & cooked the pads! :embarrest:

So it's the age of the fluid in the system that affects performance more than the heat your generating?

Car's got new fluid now but I think I'll change to 5.1 when I get the new discs & pads.

 

[obi_waynne]

Yes it is the age, and particularly how much exposure it has had to the air. Leaving the reservoir lid open is a big no no.

Good idea. Do you have braided hoses as well? These are less subject to flexing under pressure than the conventional ones.

 

[pgarner]

the heat is passed on through the callipers. when it cools it takes in more air and therefore water in my understanding.
not such to chance it

 

[HDi fun]

Don't shake the bottle either. Leave it stand for a couple of days before using the stuff to avoid the ingress of air bubbles.

If you're changing to DoT 5.1 then make sure you flush the system fully (isopropanol and lab alcohol are suuposed to be good) , and that all the seals are genuinely compatible.

 

[Loz]

HDI, I've been told that dot 5 is should be avoided because it can eat seals & 5.1 should be OK for any car. Does that sound right or are they talking cobbles?

Good idea. Do you have braided hoses as well? These are less subject to flexing under pressure than the conventional ones.

I'll fit braided hoses to the fronts when I change the discs, pads & fluid, may as well do it all in one shot . The rears can be a nightmare to change so I'm going to leave them alone for now.

 

[T9 man]

Have just had some 4 pot calipers, pads, and new discs with braided hoses fitted onto the beast within the last week, and it was a marvellous upgrade or so I thought after a quick road test?

I was asked about the brake fluid as the fitter said that the feel could be much better! I told him it was the Saab's original brake fluid which was at least two years old, and he advised me to replace it straight away with the Dot 5.1

Yes it is expensive! I paid £14.99 per litre and bought two litres to complete the job and to save some to use as a top up.

Note: This stuff is like buying car oil in that's its cheaper to buy it in the larger size of 1 litre than the top up size of 250ml if you can help it!

The difference again was amazing! Highly Recommend the upgrade, Dot 5.0 can be mixed on top of Dot 4.0 but 5.1 is designed for cars with ABS.

Hope this helps

 

[Loz]

Thanks T9
If DOT4.1 is hydroscoopygloopy, what happens to the water that collects in a system containing DOT 5/5.1?

 

[wolfmankurd]

I thought they absorbed water on purpose to keep it out of the system?

 

[Loz]

For some reason I thought dot 4.1 was Hygroscopic but the others didn't, I don't know why.

 

[wolfmankurd]

I've just had a quick google/wiki and it looks like most of the stuff is similar to water cooling addatives.
I guess for the same of computer parts and car parts they both have to be non-corrosive.

 

[Loz]

Ta, thanks for looking.

 

[T9 man]

Hi Loz, I actually asked the fitter that same question? reply was not as exact as would have liked, but he said that it will increase the feel of the pedal, but ideally you should replace it with the higher performing fluid.

He also suggested fitting the braided hoses as they will minimise the expansion due to the pressure of the applied fluid, where as the rubber ones will expand and therefore the feel of the pedal will be reduced, but i'm sure you chaps already new this! but this is still a learning curve for me!

Then the chap throws in for good measure, "if the brakes are all you have to stop the car with, then it should have the best and most reliable fluid that money can buy, what good is a car that really goes, if it won't whoa! when you want it too" seeing the wisdom in his words I bowed my head and said "put the fluid in please" :bigsmile:

 

[indiaman]

A very good bunch of replies indeed,nice to know that there are so many knowledgeable members .

 

[Loz]

Well said, thanks everyone.