Hubster, my 14s front brakes were feeling pretty weak, lever was firm but coming back to the grip when hot so I decided it was time to change fluid and bleed the brake system.

I used new DOT 4 fluid and bled a lot of air out of the system, now feels like new again and lever is very firm and out where it should be.

My question is on the black rubber diaphragm in the front reservoir. The service manual and owners manual say nothing about the purpose of the diaphragm or whether it should be fully collapsed or fully extended when servicing the reservoir.

I assume it prevents the brake fluid from sloshing around when the level is low and thus prevents unmasking the intake port to the master cylinder.

Also creates less space above the fluid to draw in moisture.

The manual says to service to the top line but when you do this and the rubber diaphragm is fully extended, it pushes fluid out as you screw the reservoir cap down. I ended up filling to the low line on the reservoir and then with the diaphragm fully extended and with the cap screwed on, the level was pushed up to the topline.

This seems like a fairly important detail omission in the service manual. Is the proper method to collapse the diaphragm completely and then service the reservoir up to the top mark?

What sayest thou Sir Hubalot?

Does it say to 'fully extend' the diaphragm?after filling and to recap?
Never saw that page..lol.Hi Kruz.

Does it say to 'fully extend' the diaphragm?after filling and to recap?
Never saw that page..lol.Hi Kruz.

No, that is precisely my point, my reservoir diaphragm is fully extended, about 1 1/2 inches in length and it appears to be permanently deformed in to that extended position, as in it is FUBAR! Looks like I need a new one as it doesn't look anything like the one in this parts breakdown ref. item 43028. Remember this is an original '06 and is now almost 17 years old, so kind of getting up there in age now.

* Last updated by: Kruz on 1/22/2023 @ 7:45 AM *

Here is what the diaphragm should normally look like (photo credit to Rook's Tutorial), e.g. completely collapsed.

I'll go out and take my reservoir cap back off and take a photo of what my diaphragm looks like, mine stays fully extended and has apparently been that way for awhile.

His brake fluid dropped quite a bit. The diaphragm extended to displace the reduce amount of fluid. Fold the diaphragm back in, fill the reservoir to just below the UPPER line, put the diaphram on and screw down the cap. Don't over-tighten the cap there are specific instructions. Expect a small amount of fluid to seep from the bleeders and the cap, take measures to prevent droplets of seeped fluid from flying onto the bike. It takes several weeks before the cap and the bleeders form a drip-proof seal. I tried tightening the bleeds a hair more but I don't think that helps much. Obviously, you shouldn't tighten the bleeds more than specced or the threads could be damaged and you have a real leak problem. I believe Grn suggested teflon tape on the threads some time back. I bet that would work but I'd be worried about tiny chunks of teflon in the MC...even though the tape does squash down to about nothing when you tighten.

Ahhhh - my poor 14.

Diaphragm acts 3 ways. One is to keep the surface airless so no condensation mixes with the Dot. Next is atmosphere so the fluid can move and remains neither locked at a vacuum suck or a push on it as pressure, but remains at a neutral or zero pressure (14.7 psi). Last is fluid loss due to pad wear and the diaphragm can follow down keeping an air bubble from causing it to leave an (air) gap.

Expand the diaphragm. Completely dry it with a paper towel. Collapse the diaphragm to install. Wipe the top ring of the master dry so both contacts acts dry. Fill to said line. Note that you have the pistons set farther out in the caliper, adding more fluid. Once it's time to replace the pads, remember to take a little out of the master before retracting the pistons back to home.

* Last updated by: Hub on 1/22/2023 @ 10:46 AM *

1:32 mark is a dried off diaphragm being collapsed and shown how cap is assembled.

From there on it's opened once and closed once. That's the trick. The ?-hook of the hose acts the drain S-nake. That means I can walk away from it, fill the master back up, continue pumping, and last pull of the lever is the nipple is closed before the lever reaches the grip.

4:04 mark is how I install the cap. Other option is cap assembly and that way. But I find the diaphragm first works best. Plus the snap of the finger to how I tighten the oil cap as well.

4:14 mark: Notice the cutout at the plastic washer. There is your 14.7 breather vent.

4:28 mark is if you sucked air at the master, or can't get a hard pedal after bleeding, you expel the air out of the master's nipple fitting.

Good stuff there Hubinator, thank you ....

What about the horror stories you hear about where the reservoir is over serviced with fluid and when the fluid gets hot and expands it pressurizes the brakes and they lock up? It would seem the reservoir diaphragm would prevent this scenario? It happened to a GSXR 1000 owner here in Dallas a while ago, he had the dealer's mechanic service the front brake reservoir and he overfilled it and endoed the bike on the freeway with brake lockup. It seems impossible with the diaphragm able to compress but Rook tells me the Suzuki's don't use a diaphragm in the reservoir?

I have no clue what brake master Rook is talking about, but went from 2001 to 2022, and it shows a rectangle diaphragm. I could take a lot of guesses on the brake flipper, but who knows? Yes, the reservoir would be squished and no way to breath at the diaphragm. The why were the brakes changed in the first place?

Pad drag and the mechanic did not catch it? Never touched the master to change brakes? Lots of variables.

Hub, you are absolutely correct, I don't know what Rook is talking about, maybe I misunderstood him but the parts breakdown on his '08 Busa shows the rectangular diaphragm in the master reservoir.

I have no clue what brake master Rook is talking about

My Hayabusa has no telescoping diaphragm in the MC. There's just a gasket. The Kawasaki design is better. Kind of weird but the busa does have a telescoping diaphragm in the rear brake MC, not the front.

* Last updated by: Rook on 1/28/2023 @ 7:17 AM *

Think about it Rook, it's a brake system with the exact same theory working against the atmosphere and has to have a diaphragm that follows the same fluid drop. This is not a busa site yes, but this is a generic part off of any hydraulic brake master on any bike. Gotta remember that man is handcuffed to nature and that 760mmHg/101.3pa/14.7psi.

This is #12 diaphragm of first year 1999 busa.
https://www.ronayers.com/oemparts/a/suz/50d40e90f8700230d8b5083f/rear-master-cylinder-model-k3

This #14 diaphragm of a 2022 busa.
https://www.ronayers.com/oemparts/a/suz/6332f7237c60483ad97a85a9/rear-master-cylinder

Next time you change fluid, just open the caliper nipple and start pumping. Watch the diaphragm of the rear's master walk down with the fluid.

I suggest bleeding your brakes and clutch yearly. Brakes are important, and on a bike it's cheap and takes an hour or two. I usually flush the lines completely, ensuring all new fluid is in the system. Vacuum pump IMO makes a lot of this easier, first suck all the remaining fluid in the reservoir. Then fill, being flushing following bleed steps. I usually end up running 2-3 reservoirs worth of fluid before calling it done.

Bleed and fill as specified and install the diaphragm collapsed. I also do what Hub says, and wipe the diaphragm completely off. As far as fill, I do not believe it is intended to have no air in the reservoir. However, I do believe the amount of air is intended to be minimal to the point of where operating the bike at maximum potentials will never introduce air into the lines. IE, say at maximum lean or even a tip over, air should never cover the reservoir's port.

* Last updated by: VicThing on 2/4/2023 @ 9:14 AM *

My ZX-14 brakes are rock hard now, I pumped two or three reservoirs full of new fluid through the system.
I need to pull the reservoir cap back off and collapse the diaphragm as it is fully extended.

* Last updated by: Kruz on 2/5/2023 @ 9:04 AM *