You may think that once your brake fluid is in the reservoir then that's all you need to do but you'd be wrong. Brake fluid is hygroscopic which means it readily attracts moisture from the atmosphere, and over time it get saturated with water to the point where it lowers the fluids boiling point. Brake fluid comes in different grades and shouldn't be inadvertently mixed without checking what grades mix with what.

Are you ready for the science bit?

You can compress a gas but you can't compress a fluid, remember that as its a useful thing to remember,
This is the principle that your brakes work on. When you press the brakes in your car you create hydraulic pressure in the hydraulic system which acts on pistons at the wheel end that actuates the brakes, imagine a bubble of air in the hydraulic system..... right what did I say up there? yup you can compress a gas but you can't compress a fluid, so if there's a bubble of air in the system then when you press your brakes instead of the hydraulic pressure acting on the pistons for the brakes all your effort is going to be used compressing the gas instead of working the brakes.

So, where does the air come from? it can come from a variety of places, worn seals etc. in hydraulic components, wheel cylinders for example but more common and even more unexpected is boiling brake fluid, this is why you should change your brake fluid at the recommended intervals, normally every 2 years for DOT 3 & 4.

What happens when brake fluid boils? okay, so your going down a hill, its a steep one and instead of using engine braking (very important in a heavy camper) and occasional use of the brakes to slow you down you go all the way down the hill on the brakes, the heat generated by the friction of the brake pads against the brake discs causes heat to build up on the disc and in the caliper, this heat is transferred to the brake fluid.
If the brake fluid is old and has a low boiling point then the brake fluid will begin to boil, when the fluid boils it creates bubbles, so when you next go for the pedal you will find it sinks to the floor with very little if any effect on retarding the vehicles speed. Not good, once cooled the brakes return to normal. This is know as brake fade, vaporized or vapor locked depending who you talk to.

So how do I avoid brake fade? by replacing your brake fluid every 2 years as VW recommend (as well as most other manufacturers) The boiling point of your brake fluid can be checked with a hygrometer which samples the fluid and boils it, the temperature at which it boils is displayed on a screen, most good garages have a hygrometer and can carry this check out for you, you can also use a refractometer which I have absolutely no knowledge of so I won't go any further! If you are in any doubt about your fluid change it, you can't take short cuts with your brakes.

How do I replace the brake fluid? My method involves removing the old fluid from the reservoir by means of a large syringe, cleaning the reservoir of debris and dirt, topping up the reservoir with fresh brake fluid and bleeding the brakes through until clean fluid can be seen coming from the bleed nipple. I use a small plastic drinks bottle with a length of clear windscreen washer pipe pushed to the bottom of the bottle with the other end attached to the bleed nipple, clear pipe is used so bubbles of air and clean fluid can be seen coming out.
If you are unsure about carrying this sort of work out then take it to your garage, most garages carry out a brake fluid change for a little as 20 GBP 2004

Which fluid do I use? DOT 4 is the one to go for, DOT 3 has a lower boiling point, DOT4 has a higher one, DOT5 is silicone based and is not to be mixed with anything else. To be able to use DOT5 you would have to completely remove all traces of anything else from the brake system which to be honest isn't worth the hassle involved, but on the plus side DOT 5 is not hygroscopic hence it is used in higher performance cars.

Below is a copy of an e-mail receive from one lucky reader, I have no experience of what was said in the e-mail so I would feel cheeky changing the details above, so I have added the mail for you to read and make your won decision as I cannot back up what was said.

Re: Brake Fluid

Dear Simon
I know this is somewhat picky, but I think that your piece about brake fluid is slightly mistaken.

Your description of brake fade might be better described as "fluid fade", true brake fade occurs when the pad/disc temperature exceeds the capabilities of the pad material to exhibit sufficient friction, indeed I first discovered this unfortunate scenario using mechanical brakes, (by which I mean no fluid involved), of course although the effect is similar, not much braking, the symptoms will be different.
If you have what I describe as brake fade, then the pedal (or lever, if you're riding), will not soften but will just be ineffective, and in my experience judder is common, if it's "fluid fade" then of course the brake will "pump up" (although probably not to much effect), as compressible gas expands in the system.
It is also possible to get both inconvenient situations at the same time, so of course your advice is quite correct, regular brake maintenance and fluid changing must be carried out, but then as my father said "Who needs brakes? They only slow you down". Again I am aware that this is a very small point, but it might be of interest to those reading the information you so kindly supply on the excellent Club 80-90 web site.

Yours Sincerely
Ian Patterson

Ian@footprint.freeserve.co.uk

cheers for the feedback Ian, nice to know people read them!

see don't just listen to me! if you find any faults then do as it says below and let me know and I will be happy to change said page(s) and let everyone else benefit from your knowledge.

All information contained within this website is intellectual property of Simon Baxter unless otherwise stated.
Copying is prohibited unless prior arrangements have been made.

Site sponsored by Brickwerks and SyncroSpares UK

Talent borrows, Genius steals, shit copies. MitDR