Forum Chat #36 – Why change brake fluid at each annual service?

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Welcome to Bamford Rose and another form chat. This week, we’re going to talk about a subject I quite often see. There’ll be a post somewhere on a forum saying, ‘Where’s the cheapest place to service my Aston?’ I always do think that’s totally the wrong question. Do you really want the cheapest place servicing your Aston? I can understand the spirit in which the post was made, but it’s not the right viewpoint. Also, hand-in-hand with figuring out the cheapest place to service your Aston is the question that keeps arising. Come on you don’t need to keep changing brake fluid every year. And some people say yes, some people say no and I just wanted it to add data to the subject.

So, brake fluid is a hygroscopic, which means it absorbs water over time. And we can pop up a graph here which shows when the brake fluid was new and when the brake fluid has absorbed water. What that water ingress content means in terms of how it will affect the performance of the fluid? And this is what’s known as a dry boiling point and a wet boiling point. So, the dry boiling point is when the brake fluid is brand-new out of the bowl. and there’s in the spec depending on the grade of the fluid, there’s a figure for that. The better the fluid, the higher the temperature. Then the wet boiling point is when the brake fluid is absorbed a certain percentage is 3.7% water. It’s a measurement of how each fluid behaves at that level of water ingress.

And as water is the water percentage by volume of water increases the boiling point of the fluid comes down. And you can see on this graph here that the lower grade fluids as soon as they hit you know 8% the boiling point of the fluid is practically the same as water. Obviously, in the Aston we use Castrol React Super DOT4 and already this is a really high spec brake fluid. We’ve got a dry boiling point of 260 degrees and a wet boiling point after it’s increased the percentage of volume by water of 185 degrees. In the race cars, we use a synthetic fluid and that’s Castrol SRF and that has 300 degrees dry boarding point with 270 degrees wet.

So, after that fluid has absorbed water, the reduced boiling point is still better than the fresh out of the bottle Castrol React Super DOT4. Some customers have bought some of their own brake fluid in, thinking that it’s better. There’s been a Motile 660 fluid that we’ve had in has got an extremely good dry boiling point 328 degrees. It’s wet is 204, so it’s not that far off the Castrol React Super DOT4. So, just a word of caution there when you’re selecting a brake fluid which you think is better than the Castrol React Super DOT4 standard, just check out the dry and wet boiling points to see that it actually is better fluid.

So, there is a naming numbering convention, DOT 3.4 and DOT 5.1 Oral Glycol-based, DOT 5.0 is synthetic and you cannot mix them. So, if you want to change from Glycol to a synthetic, you have to flush out. It’s probably gonna take two liters to do that properly. So, that’s one part of this form chat different brake fluids and why you might change them. So, the next is coming on to servicing. So, every year every service here, we do what we call a Caliper Bleed. Where there’s the inner bleed nipple out a bleed nipple and do about five seven pumps on each bleed nipple. And regardless of whether the fluid needed changing, then when the customer drives away after service that caliper bleed gives that just service feel. You know the brake pedal is going to feel a little bit sharper.

So, I don’t think any garage out there would annually completely flush out from master cylinder through to the ABS unit through to each caliper, completely flush out all of the fluid. I think a brake fluid service or what they do at point of service is what we would call the Caliper Bleed. And certainly, by the time you’ve done you know seven or so pumps on the inner and outer bleed nipple on all four corners, you’re gonna flushed through from the ABS unit down into through the calipers. So, the fluid in the master cylinder, its brake fluid. Its hygroscopic, it’s going to be absorbing some water. The only side effect of that is that it’s going to reduce it boiling point.

Now try and do some research on the web, as soon as you open a break bottle how quickly does that industry-standard water by volume absorption level take to ingress. And it’s quite difficult to find that out. There are some places that say it happens super-fast like within a month or something like that. Other places say typically it’s two years. It’s difficult to find that out. But for those people who are thinking at service it’s just completely unnecessary to flush out and we know the Blake brake fluid. As I say, I don’t think that happens. It’s just a caliper bleed and that takes minutes and it makes your brake pedal feel nice and fresh after each service. But go back to the start and the whole philosophy of asking this question to drive down the price of servicing.

Well, you don’t really want to thrift that minor brake pedal, brake caliper bleeds out of it. Not least, because every year you’re then undoing your bleep nipples and it means that they shouldn’t really seize if you left your bleed nipples without touching them for a couple of years. Probably going to find that caliper. It’s gonna seize in the caliper then you have to remove the caliper and it’s going to be a bit of a tricky engineering job. Not impossible to machine out the bleed nipple, and then any plan of not touching your brake calipers. Because you thought you were saving money by just completely blowing out the water, because it’s gonna cost way more to rework the caliper.

So, you know I just let garages get on with what they know best which is how to make the car feel like it’s just been serviced, everything’s nice and fresh. And if you think the brake caliper, a brake bleeder every annual service is over the top. I mean we could get asked not to. It wouldn’t change the price of the service and we’d still do it anyway. Because we know that there’s gonna make the car have that just service feel.

So, I hope that’s given a little bit of data behind what’s happening to the brake fluid, and see you in the next video.

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