master cylinder diaphragm

[deltarider]

Just serviced the brake fluid and noticed that the diaphragm on top in the master cylinder has a small tear. How serious is that?

[McCabe-Thiele (Ted)]

Not sure what the diaphragm is for -  to minimise moisture ingress or keep the fluid in place when cornering.

[K2-K6]

It's for expansion space in hydraulic system when used/heated in operation,  if not there the pressure build would just squeeze the piston and apply the brake until it cools (cars vent to air as they don't ordinarily get too out of level like a bike cylinder) with the diaphragm being part of design concept to work effectively.

Fluid below is seperate from air above to prevent risk of air being near the fluid entry port to operating piston and bleeding air into line supply.

Needed to function correctly on any brake that's tipped up during normal use. Think about parking on side stand with  bars turned to the left.

[Bryanj]

Depends where the tear is but they should be easily found as same one fits a lot of Hondas

[robvangulik]

Just replace it, according to cmsnl this part fits 4875 models, an original Honda one costs a staggering €6,00, and the non OEM alternative is €2,50.
Gives you the opportunity to refresh your brake fluid at the same time, as you stated recently that's over 10 years old.
45520.300.000
At Roukama €7.00

[Bryanj]

Dave Silver has them or just order from your local dealer and not pay postage

[Bryanj]

I think you will find it is a reinforced synthetic rubber

[deltarider]

I'll get one. I just wondered if I could ride the bike to go and get one.
@Rob. Replenished the brake fluid yesterday. The old just a tad darker, but not much. Renewed the bleeder that needed locking pliers to be freed. With an expert I closely examined and tested my brakelines. We couldn't detect any sign of wear. Although you can feel it in the lines when you apply the brake, it is minimal and I don't expect it to be less with a new OEM line. In general people refer to these lines as rubber. I wonder what material the inner tubes are really made of. I don't think it's rubber.

[deltarider]

I think you will find it is a reinforced synthetic rubber

Aha, I'll try to find the specific name used in the industry.

[McCabe-Thiele (Ted)]

Just as a matter of general interest flexible brake lines had a in-use life of 5 years back in the 1960's- most UK manufactureres recommended replacement after 5 years on cars as routine maintainance.

The life and quality of materials has improved substantially since then but the life span does not seem to be mentioned for modern materials. I suspect climate & mileage make this difficult to quantify. I had a front brake hose fail on my Carlton - turned out to have missed a factory recall as the top & bottom steel sections had been assembled upside by the suppliers down causing abrasion and failure. Luckily despite the hose inflating like a baloon the dual circuit avoided an accident

I tend to run older vehicles these days but I do replace the flexible wheel hoses & rear axle hoses where fitted as a matter of routine once they are 15 years old or sooner if the end fittings have rusted or if they have stone type damage.

Personally I see 10 years on a motor cycle as a maximum even if visually sound - there are a lot of replacement products of inferior quality that I would not buy unless its a known brand & not a fake product.

[deltarider]

[...] there are a lot of replacement products of inferior quality that I would not buy unless its a known brand & not a fake product.

That is exactly my concern. My brake operates fine, there's no sign of wear of the outer protective tube and I have no indication that the inner tube where the fluid passes, can age. Brake fluid is nasty and aggressive stuff. It seems to me the inner tube is of such material that it can withstand that and there's no danger the brake fluid will eat it. If it would, that would be happening much sooner than any period in the maintenance scedule could prevent. What I can understand, is that manufacturers advice to nonetheless replace the tubes after a certain period of time to be a step ahead of possible wear of the outer protective tube. If my outer tube is not damaged, shows no sign of age whatsoever, I conclude nothing is wrong and I prefer to leave things where they are.

[McCabe-Thiele (Ted)]

In the main I agree with you comments - I see the main weak points as the end sections where the hose is attached to the metal fittings.
The hoses on my 400 are well away from the road unlike on a car.

That said there will be in a finite period where the hoses will deteriorate due to a slow break down of the material structure caused by fluid & uV degregation - add to the mix the changes in pressure to the hose walls and that they are of composite construction they will loose optimum performance over time.
 Brake seals have improved beyond all recognition when I look back at how often car wheel cylinders would last - plus the black suspended streaks you can see in old brake fluid.