Corvus wrote:Tapio: what make the old fluid dirty and black?
Anybody's guess is as good as mine. I have no clue.
Maybe a previous owner put some fluid in there that really wasn't meant to be used as brake fluid? And it started dissolving O-rings and seals. The brakes work perfectly, though.
Now, iron corrosion comes in three shapes: FeO, FeO2 and FeO3. One of these are dark grey, or black. Don't remember which, but it's the one that is commonly put on gun barrels, to prevent rusting.
Maybe steel parts in the ABS/servo system had started to corrode?
Archie wrote:I had a GSX-R1100 that had never had the clutch fluid changed, and one day when riding it the clutch started slipping when the engine got up to temp.
It turned out to be the fluid, as changing the fluid completely fixed it. It seemed illogical, and I concluded that the clutch hydraulic system must be sealed, unlike the brake. I.e. it is not open when released, and cannot adjust for wear.
On a similar theme, old bevel drive Ducati brembo master cylinders are usually trashed by moisture in old fluid when they have been stood for a few years. They get quite substantial corrosion in the bore.
I always assumed the clutch side worked just like the brake, but maybe with more travel? But I'm not 100% certain.
Corvus wrote:Tapio: what make the old fluid dirty and black?
Anybody's guess is as good as mine. I have no clue.
Maybe a previous owner put some fluid in there that really wasn't meant to be used as brake fluid? And it started dissolving O-rings and seals. The brakes work perfectly, though.
Now, iron corrosion comes in three shapes: FeO, FeO2 and FeO3. One of these are dark grey, or black. Don't remember which, but it's the one that is commonly put on gun barrels, to prevent rusting.
Maybe steel parts in the ABS/servo system had started to corrode?
Etched in my memory is losing my back brake while descending in the Pyrenees in 1989. I'd changed the front brake fluid on my recently bought CB750F before the trip but for some reason didn't bother with the rear. Realised it had failed when I almost rear-ended a mate as we stopped for a break. Lever travelled to the stop with no apparent effect. Thank God I'd bled the fronts Normal service resumed once everything had cooled down.
Whatever the exact mechanism the result is in no doubt.
Regarding brakes, I've come across corroded master cylinder return springs.
No, no, , please don't push me that way! I don't know!
I do have some experience with fretting corrosion though, on spines. With some freeplay, you get brown, powdery stuff, from spline wear. If spines are dry.
But i've done tests with the same splines with grease on them and the grease turns from yellow to black (from contamination), not brown, with wear. Why's that? I don't know. I'm thinking it's because lack of oxygen.
Nobody knows. The ”mechanism” (is that the right word?) of fretting corrosion is not known to man.
Google it all you want.
you'll find a lot of papers on it, but none expaning what is actually happening
Last edited by Tapio on Tue Mar 10, 2015 9:18 pm, edited 1 time in total.
Etched in my memory is losing my back brake while descending in the Pyrenees in 1989. I'd changed the front brake fluid on my recently bought CB750F before the trip but for some reason didn't bother with the rear. Realised it had failed when I almost rear-ended a mate as we stopped for a break. Lever travelled to the stop with no apparent effect. Thank God I'd bled the fronts Normal service resumed once everything had cooled down.
Whatever the exact mechanism the result is in no doubt.
Mike
Some rear brake hoses are routed (unavoidably) near to exhausts. Plus rear calipers seem to be often lower quality than front ones, plus they are more in the line of fire for crud. Seems to me that all these factors maybe tend to feed heat into the system before the actual braking even starts.
Regarding brakes, I've come across corroded master cylinder return springs.
No, no, , please don't push me that way! I don't know!
I do have some experience with fretting corrosion though, on spines. With some freeplay, you get brown, powdery stuff, from spline wear. If spines are dry.
But i've done tests with the same splines with grease on them and the grease turns from yellow to black (from contamination), not brown, with wear. Why's that? I don't know. I'm thinking it's because lack of oxygen.
Nobody knows. The ”mechanism” (is that the right word?) of fretting corrosion is not known to man.
Google it all you want.
Yes, I too have witnessed the effects of fretting corrosion. There seems to be a need for relative motion (at high frequency?) between the two affected surfaces?
Regarding brakes, I've come across corroded master cylinder return springs.
No, no, , please don't push me that way! I don't know!
I do have some experience with fretting corrosion though, on spines. With some freeplay, you get brown, powdery stuff, from spline wear. If spines are dry.
But i've done tests with the same splines with grease on them and the grease turns from yellow to black (from contamination), not brown, with wear. Why's that? I don't know. I'm thinking it's because lack of oxygen.
Nobody knows. The ”mechanism” (is that the right word?) of fretting corrosion is not known to man.
Google it all you want.
you'll find a lot of papers on it, but none expaning what is actually happening
Black Magnetite
If I am ever on life support - Unplug me......
Then plug me back in..........
Anyhow. Who's to say that the temperature won't reach the higher level? Going by my 250psi (nobody has challenged it, so I'm going with it) and using blackal's graph, we need a temperature of 170 C. I suspect heavy braking from high speed could generate that.
Who's going to do the calcs for the amount of heat generated?
Let's say it boils. On a relaxed lever, that would allow the fluid to expand into the res, via the tiny hole. Once the vapour returns to water, and the fluid volume contracts, how long will it take, bearing in mind we now only have atmospheric pressure acting, for the displaced fluid to find its way back?
Well, I replaced my iABS fluid in all four circuits today without a problem.
It's a bit nerve racking the first time but I'd encourage anyone to give it a go, it's really no harder than any other modern brake system.
-- Watch the youtube vid
--Seems to me you need to have more than 500ml of new fluid to hand.
--Be very careful of the "squirt" out of the open front brake reservoir when you first squeeze the lever.
--Release the front brake lever slowly after each bleed stroke to prevent air getting sucked into the master cylinder by a sort of cavitation type effect.
--Obvious I know but, keep the fluid levels well topped up and do it all patiently and methodically.
Ok, Mitch, OK. I know I got a bit carried away there, but things like seize bleed nipples in ally callipers and abs units scare the living daylights out of me .... so I was feeling a bit light headed.
BTW that reminds me, are you aware of the dangers of riding whilst feeling light headed ....?
Normal service resumed once everything had cooled down.
