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My 1984 (Brembo) R100S has dual discs. Can anybody estimate how much brake fluid it would take to flush and fill my system?
Thanx

No...

and I wouldn't accept that anyone else does either. There are too many variables. Buy a big bottle and keep flushing till the fluid is clean and there are no bubbles. Might take a hundred cc, might take a litre.

Rob

I've never used more than a couple of hundred cc on single disk system which should be adequate if the fluid has been changed often enough. I like to buy the Ate fluid in a tin because Ate say it seals better and I can save what's left for use on the car.
I don't accept that brake fluid should never be stored. If it lasts two years in a vented reservoir exposed to all climates then it will last that long in a sealed tin on the shelf. Ate claim a 5 year shelf life on unopened tins.

barryh said: " If it lasts two years in a vented reservoir exposed to all climates then it will last that long in a sealed tin on the shelf."
Brake hydraulic systems ARE NEVER vented to the atmosphere. There is always some form of bladder or accordion seal to compensate for the fluid level dropping as pads wear and to prevent atmospheric contamination. That's why fluid doesn't spill out when your bike is tipped over.

The cover itself will have some form of ventilation to allow air to enter above the rubber seal so as not create a vacuum.
This vent can be almost invisible; a pinhole in the cap, or a discrete passageway that vents to the edge of one of the screws that holds the cover on.
Take off your cover and closely inspect the insides. This vent is VERY OFTEN blocked and goes unnoticed because removing the cover to top up the fluid when your brakes feel spongy releases the vacuum so everything works as it should until further pad wear creates vacuum again, and sponginess again, weeks or months later...

I like buying brake fluid in plastic bottles because I can squeeze the air out before resealing the leftover for later use.

FWIW, my 'practice' is to keep the left over fluid for top up but to buy new if I'm going to flush or refill the system... partly to make sure I'm not using fluid that is years old and partly because it ensures that I have enough fluid for the job.

I would say that 250ml is more than adequate for a flush when things go according to plan but I have had systems which, for whatever reason, have given difficulties and I've used far more than I expected. If you've got a shop round the corner, that's not too much of an issue but when the nearest stockist is miles away, it can be, to say the least, frustrating.

Rob

Thanx all, as usual, great perspectives based in logic!

I cant comment on how much fluid you will need but, last fall I replace the front brake lines on my 81 R100RT. I used a disposable veterinary syringe to slowly push the fluid up from each caliper. No waste or bubbles! They can be bought at a farm or tractor supply store.

PAS wrote: ↑Sat Jan 16, 2021 10:17 am I cant comment on how much fluid you will need but, last fall I replace the front brake lines on my 81 R100RT. I used a disposable veterinary syringe to slowly push the fluid up from each caliper. No waste or bubbles! T

That's how I do it for a complete fill from empty.

For a routine fluid change I do a gravity bleed. Evacuate most but not all of the fluid from the reservoir and refill with fresh. Then attach a length of tube to the bleed nipple and open it. Fluid will flow under gravity with zero risk of introducing air. It works particularly well with the newer low viscosity DOT 4+ fluids which I use with the same method on the car.

Dot 4 + also called Dot 4 LV, Dot 4 super, DOT 4 plus and there are probably others. They are all thinner versions of Dot 4 with a maximum viscosity of 750 cSt @ -40 Deg C. compared with 1800 cSt@ -40 Deg C for normal Dot 4. They are intended for use in brake systems with ABS etc to speed up performance in cold weather.

The reason for the various names is that there isn't yet an official Dot standard for low viscosity Dot 4 brake fluid so they are all just marketing terms. It’s very confusing that each manufacturer thought it appropriate to invent their own name. Often you’ll see them marketed as DOT 4 + class 6 (or any of the other variations followed by class 6) which is a clue to what they really should be called. This refers to the European ISO standard 4925 class 6 which specifies the low viscosity rating of 750 cSt @ -40 Deg C.

Having to fudge the North American standard DOT 4 with plus and super etc is a bit of a mess but I suppose the manufactures thought that if they marketed their brake fluid as "ISO standard 4925 class 6" they wouldn’t sell any.

Why didn't Dot 5 (silicone) brake fluid gain acceptance? It's not corrosive, doesn't absorb water, seems to me it should be a better product for street bikes.