Brake fade + vibration = Baws!

[Mr. Dazzle]

Yeah, exactly...so when the brake fluid boils the bubbles you get are made from the brake fluid itself, you don't need any air to be there. I.e. a non-leaky fully bled system can still get bubbles in it.

FWIW people often say that water in brake fluid boils, which is kinda does. it's more accurate to say that water dissolved in the fluid lowers the boiling point of the combined fluid/water mix. That's the primary reason you're supposed to change brake fluid, the fluid 'soaks up' water from the air over time and it's boiling temperature drops.

[Skub]

I had an RCS19 on my 10R for over 10 years with no bother,I'd be looking at things that make the fluid boil,either pad material,or slightly warped discs.

The stock calipers on my 04 10R were ok for road use,but not track. The caliper pistons were made too thin,so the heat from the pads transferred too quickly and boiled the fluid after a few laps,then brake fade city. I swopped them out for newer Blade calipers,which worked fine.

[wull]

With warped discs, you will feel a pulsing through the lever. On my K9 and L3 it happened. It's not quite warping, more the bobbins sticking. I improved things by taking the disks off, then soak the bobbins in WD40 then give them a reet good twatting with the big 'ammer.

The best braking performance improvement is to use an Accossato Master cylinder. I used GSXR1000s on track 15 years and never had brake fade using the Accossato M/C.

I even have one now on my 2019 bike.

That was another thing I tried, freed them off and it made sweet fook all of a difference.

[Yorick]

With warped discs, you will feel a pulsing through the lever. On my K9 and L3 it happened. It's not quite warping, more the bobbins sticking. I improved things by taking the disks off, then soak the bobbins in WD40 then give them a reet good twatting with the big 'ammer.

The best braking performance improvement is to use an Accossato Master cylinder. I used GSXR1000s on track 15 years and never had brake fade using the Accossato M/C.

I even have one now on my 2019 bike.

That was another thing I tried, freed them off and it made sweet fook all of a difference.

Spin the wheel and brace a screwdriver against the fork leg with end almost touching the disc.
You will see if it runs out of true.

[Count Steer]

Yeah, exactly...so when the brake fluid boils the bubbles you get are made from the brake fluid itself, you don't need any air to be there. I.e. a non-leaky fully bled system can still get bubbles in it.

FWIW people often say that water in brake fluid boils, which is kinda does. it's more accurate to say that water dissolved in the fluid lowers the boiling point of the combined fluid/water mix. That's the primary reason you're supposed to change brake fluid, the fluid 'soaks up' water from the air over time and it's boiling temperature drops.

How does the air get in contact with the brake fluid? I suppose there's a small amount in the reservoir? It seem odd that water from the air gets in without the air getting in IYSWIM.

[Le_Fromage_Grande]

Brake fluid absorbs water from the air, but it's only the amount of air at the top of the reservoir (hopefully), so usually a tiny amount of water unless you're constantly taking the lid off your reservoir or it isn't sealing.

[Mr. Dazzle]

Imperfect seals basically, yes. And imperfectly dried fluid that went in to begin with...how well do you control the storage and handling conditions of the fluid in your garage etc? They put load of margin in of course. But in 'serious' applications* you'll go to much greater lengths to make sure you put dry fluid in and keep it dry.

The rubber lines and plastic reservoirs won't be water tight either, strange as it sounds. Polymers and elastomers are notorious for taking up water, which they can then transfer into whatever is inside them.

*not sure how much more serious you can get than not being about to stop your bike though!

[Count Steer]

If the seals aren't perfect, wouldn't you notice the air ingress before the water absorption? (Or fluid going the other way).

I suppose for storage it could be kept in a flexible container and squeeze the air out and you could put a whiff of dry gas into the reservoir when you cap it.

(I know that you're supposed to change the brake fluid at intervals but never thought about why before. On a big vehicle that's quite a lot of fluid to very little air/water. I'm surprised it's such a big factor).

[Mr. Dazzle]

I don't know the relative contribution of the various factors. Although now that I think about it, I'd bet that it's actually the permeability of the reservoir and pipe linings that's the major source. They can be air tight but still let water through! Anyone with a swollen plastic fuel tank knows how water tight plastic actually is.

As I said, they'll also put loads of margin in. The recommended interval will be based on someone who lives in Singapore but also manages to park at the top of a mountain every time they stop.

[Le_Fromage_Grande]

My FZ has 36 year old brake fluid in the back brake, still works.
And air from 1985 for that matter, but it's special racing air.

[Count Steer]

I don't know the relative contribution of the various factors. Although now that I think about it, I'd bet that it's actually the permeability of the reservoir and pipe linings that's the major source. They can be air tight but still let water through! Anyone with a swollen plastic fuel tank knows how water tight plastic actually is.

As I said, they'll also put loads of margin in. The recommended interval will be based on someone who lives in Singapore but also manages to park at the top of a mountain every time they stop.

Just been down the internet rabbit hole....reservoirs are usually HDPE which is v v good water-wise (found someone trying to 3D print one!). My bet is it's anything flexible that is the issue, so the more stainless tube and the less flexible bits the better. I also bet that 99.9% of the fluid changed at service intervals is near perfect!

[Yorick]

All Suzuki master cylinders have a little rubber boot thingy to eliminate any air.

[Mr. Dazzle]

I also bet that 99.9% of the fluid changed at service intervals is near perfect!

Well yeah, there's definitely that...but if the bit which is right next to the piston, which is the hottest bit (and also the bit closest to the outside world) is the 0.1% you've got a problem no?

Manufacturer's like to keep their arse fully covered too.

[Count Steer]

All Suzuki master cylinders have a little rubber boot thingy to eliminate any air.

Doesn't the front one* have a little window in to show the level? How does the rubber boot thingy work? I thought the level was where the fluid and the 'not-fluid' met. Or does the window have to show all fluid rather than a level?

* the rear looks like a standard HDPE plastic pot.

[KungFooBob]

There's been a rubber 'boot' inside the top of every reservoir I've ever refilled!

The 'boot' expands to fill the void.

[Yorick]

All Suzuki master cylinders have a little rubber boot thingy to eliminate any air.

Doesn't the front one* have a little window in to show the level? How does the rubber boot thingy work? I thought the level was where the fluid and the 'not-fluid' met. Or does the window have to show all fluid rather than a level?

* the rear looks like a standard HDPE plastic pot.

#13 is the boot

SU4_-2030385.png (24.14 KiB) Viewed 169 times

[Count Steer]

There's been a rubber 'boot' inside the top of every reservoir I've ever refilled!

The 'boot' expands to fill the void.

I must pay more attention when I take things apart. It appears that all mine have had them too...I've even found the spare parts numbers.

[dern]

I found this little chart on a subaru forum...



With everything in A1 condition new brake fluid will go in the bike as 'pale straw' in colour. If it gets to 'pale gold' I'm changing it. I don't know how it degrades (some water, some heat) but it's piss easy to change and not worth compromising on.

For road I'll change my brake fluid every spring. On the track bike I may change it 2 or 3 times a year.

[Yorick]

I found this little chart on a subaru forum...



With everything in A1 condition new brake fluid will go in the bike as 'pale straw' in colour. If it gets to 'pale gold' I'm changing it. I don't know how it degrades (some water, some heat) but it's piss easy to change and not worth compromising on.

For road I'll change my brake fluid every spring. On the track bike I may change it 2 or 3 times a year.

I got my bike serviced every 2,000 miles for track use. Always asked for fluid changing.

[dern]

I think that my paranoia results in me changing the brake fluid roughly every 500 miles or 3 track days. It's been heavily influenced by running straight on on the long straight at Bedford due to brake fade