According to my Renault workshop manual (the one supplied to Renault dealers, not haynes etc) A SYSTEMATIC PERIODIC REPLACEMENT OF THE BRAKE FLUID IS NO LONGER NECESSARY
The emphasis in bold is the manual's, not mine.
For good measure, this is then re-stated further down on the same page and reasons are given as to why no systematic change is necessary.
This manual is for a 1989 Renault 19 which uses Dot 3 or Dot 4 Brake fluid.
Are Renault wrong? It seems strange that they would make such clear and unambiguous statements in official documentation about a safety related system if they hadn't proof to back up these statements.
I'll bet they do. Further more, the manuals are nearly always superceded by updates to service check sheets and bulletins. You will often find they never reprint a manual. It may be that the service check sheet stipulates a test of the fluid, possibly at more frequent intervals than the original schedule.
No, it doesn't propose testing the fluid. All it says is that brake fluid absorbs very little moisture after the first few months running from new after which time the water content stabilises. It states that the braking system is designed to take account of the brake fluid in this condition, so no systematic change is necessary. It goes on to talk about the design of the pistons, cylinders and calipers and dismisses the possibility of vapour lock occuring. It advises that brake fluid must be changed in the event of extensive work being carried out on the braking system and it also stresses the type of brake fluid which must be used and warns against mixing incompatible fluids.
Now interestingly, Renault Dialogys (electronic manual from around ten years later) states virtually the same thing except the line about "no systematic replacement" is removed and in its place is a statement about replacement intervals. However it is not nearly as strongly worded as the statement on the original manual and to be honest sounds more like an ass covering exercise than anything else.
Out of interest what typer of fluid do they use? It could be they are using the newer Dot 4 Super. Although I doubt it, as this is supposed to be even worse for absorbing moisture.
I cannot see how they could not change the fluid, as the fluid is the same for everybody.
I don't know Andy, however I will say this - I'm now convinced that there is an awful lot of BS and misinformation floating about on the subject of brakes and brake fluid. It seems that Renault, a big manufacturer, is unsure about whether brake fluid needs to be changed or not and how often. Yet there are numerous armchair experts who are shooting their mouths off on this subject and stating their opinions as fact when the truth is they don't actually know.
I don't know myself either but at least I know that I don't know :) When I see a proper, independent, scientific study on the effects of changing/not changing brake fluid including effects on brake performance, corrosion and moisture content then I'll make my mind up.
It's funny how there are numerous myths doing the rounds when it comes to car mechanics. Like when people advise you to "turn your heater to hot when filling your cooling system" when you've got a modern car with an air blending heater :)
In that case it will need changing every 2-3 years.
Hmm, well I'm hardly an armchair expert, but I can tell you that all brake fluid is 'hygroscopic', and it will absorb moisture from the atmosphere. For sure the possibility of moisture actually making down to the cylinders is low, but it can happen.
I've certainly seen enough failed wheel cylinders to know that corrosion does indeed occur in these components, although it does tend to be more from the outside in. I have also changed the fluid on enough systems to know that the fluid gets to a condition that it cannot possibly work as well.
The trouble is you will probably never need to find out the condition of your brake fluid, it will only ever be known when you need the brake the most - i.e. in an emergency.
In these conditions it is possibly to attain many hundreds of degrees of temperature in the friction linings, and even if only a small amount of this gets to the fluid it will boil any moisture that is there.
It might even be that for a very short time this effect might even help you to stop, as the pressure build up might assist the forcing of the pads against the discs. Until the moisture turns to super heated steam of course then you will get a gas, and it doesn't take much thought to work out what that means.
Yes, but it doesn't do any harm does it? There are still a lot of heating systems that use the old water valve system.
Likewise, it doesn't do any harm to replace any fluids periodically, even if they don't need doing.
I've been watching your threads about brake fluid with interest. Now for some facts.
Brake fluid is hygroscopic, ie. it absorbs moisture from the air. As the brake fluid does not remain constant in the reservoir, the reservoir has to be vented. Now with temperature changes, this air in the reservoir is forced out when it warms up, and when it cools down fresh air from the atmosphere is drawn in. This fresh air contains a small amount of moisture, which is then absorbed by the brake fluid. This is a very slow process, but it does happen. Now as the brake fluid is hygroscopic, it absorbs this moisture, with the result that it slowly lowers the boiling point off the brake fluid. Under normal braking, this wouldn't really be considered a problem, it;s only under prolonged braking, that brake fluid is likely to boil. The problem is, that once the brake fluid has absorbed as much water as it can, any excess water sinks to the lowest point off the brake system (ie. calipers), where it drastically lowers the boiling point. This can result in brakes failing under moderate use.
The changing of brake fluid is a preventative measure, just like changing your engine oil is. You could run an engine on the same oil for far longer than the manufacturer recommends (modern synthetic oils have been shown to last in excess off 250,000 miles), and might never have any problems.
There is a major exception in the case of synthetic brake fluids (DOT 4 super + DOT 5). These are not hygroscopic. Although they have a far higher boiling point than conventional brake fluids, any water that enters the braking system using these fluids, will sink to the bottom off the system. With the exact same results as excess water in normal brake fluids.
moray
One last thing. The bit about turning the heater to hot when bleeding a car is very valid. Although it has no effect on coolant flow in a modern car that uses an air mixing system, it is a good sign that the coolant system is fully bled when hot air is coming out the heater. The heater matrix not working is quite often one of the indicators off any air in the cooling system. Also, could you tell simply by looking if a car used a coolant valve or air mixing to control the heater?
I agree. I've have seen a number of cars with leaking cylinders which were badly corroded from the outside. Have seen this even on vehicles where the owner has insisted that the brake fluid be changed every 2 years
It makes sense that moisture would have some effect. But how much of an effect? As I said earlier, according to Renault the effect is insignificant and vapour lock is unlikely "even when the brakes are intensively used under mountain driving conditions" (direct quote)
It probably wouldn't do any harm to change brake fluid at regular intervals even if the manufacturer says it's not needed. But who knows? If work is done sloppily, it will undoubtedly do more harm than good. Even if it is done carefully, according to procedure and using appropriate bleed equipment who knows what effects it might have. Each time it is done, air will get into the brake lines and has to be bled out. Who knows what kind of effect (oxidation?) this could have?
The message from "brian doyle" contains these words:
I suspect that the demise of rear brake cylinders is mostly caused by condensation aggravated by the hygroscopic nature of brake fluid. Rear brakes don't get as hot as fronts, nor is the layout of drum brakes as likely to warm the hydraulics like fronts do.
Perhaps taking 'em apart and greasing 'em from time to time would be a good idea. Or at least checking that the rubber dust-shield isn't perishing. I've got to do something about the back brakes on the Montego 'cos the handbrake's not wonderful and it squeaks for a hundred yards after standing.
The message from "Moray Cuthill" contains these words:
Worse - you can't get it out by flushing the system through with fresh fluid because the bleed nipple is at the top of the caliper to release air, not at the bottom to release water.
With the hygroscopic fluids there's little chance of actual liquid water being present - it'll be dissolved in the fluid and thus capable of being flushed out.
Porlock hill in Somerset is the tester. I experienced brake fade of the highest order coming down there in a Ford Fiesta many years ago, with my wife and two kids in the car.
That taught me NOT to try and cut corners on brake fluid changes - I had ignored the warnings for a few weeks and put off a fluid change due to the fact that I "had better things to do than mess with the car" - luckily we all survived to tell the tale (smell it? I was sat in by the time we reached the bottom of THAT hill) and boy, what a shellacking I had from SWMBO.
You ought to try it in an auto. You can guarantee that one locked gear is too high and you hurtle - the next one down is too low and you're screaming along - at 5mph.
Take it from me, it was bad enough with an "ordinary" box and I was afraid to change gear in case I missed the next lowest one, we survived because the handbrake held in a partially on position - tried it full on but the back-end decided to try and overtake the front. when we finally got to the bottom, the brakes took over two hours to cool off enough for me to travel the 150 miles or so back home.
The very next day the fluid was changed - my nerves couldn't stand another boll****G from the other half (I had it all the way home non-stop and even the kids were on her side chipping in the odd comment here and there to keep her in a foul mood :-( ).
That was more hair-raising than the trip down Porlock (always avoided it since that little scare) :-)
Nice image that. Did you come down the narrow road that drops into Church Stretton with the water station waaaay down to the left somewhere? I had a front tyre blow on me there once when I holed it on the rocks avoiding a sheep. As the Germans would say, a damnnearshittenselfen moment.
I've never understood why car makers don't do as my old Suzuki bike did. It had a cone shaped membrane that sat over the fluid reservoir and sealed as you screwed the top down. Air sucked in through the cap sat above the membrane - DOT4 underneath. As the pads wore down and the pistons came out the cone got sucked down and inverted itself into the reservoir.
Swapping the fluid after three years and it looked nearly as clear as fresh stuff out the bottle of new stuff.
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