Brake fluid question

[ITSv40]

Originally Posted by canis

Still a bit confused. You press in the diaphragm, and go to put it back on the reservoir, it pops out again just in time for you to reach the fluid where it hits the surface and displaces a load all over the sides. I'm not getting this, am I?

The diaphragm is pushed back into the cap before fitting the cap to the reservoir - as per attachment. When in this position the diaphragm is self supporting and does not come back out of its own accord. When the cap is screwed onto the reservoir the diaphragm sits above the brake fluid, creating an airtight seal in the reservoir and the braking system, thus eliminating moisture contamination of the brake fluid. When the pads wear and the fluid in the reservoir drops it creates a potential vacuum pulling the diaphragm down towards the fluid. Air enters the vent and fills the void between the diaphragm and cap keeping the brake fluid at atmospheric pressure. Hence, if the fluid level has dropped the diaphragm will be out of the cap when the cap is removed and needs pushing back in for the whole process to repeat itself.

As I said earlier: quite an ingenious system to keep the fluid free from moisture contamination. If your cap is not operating in that manner, it needs replacing

[canis]

Ah, right. Gotcha. They should explain this stuff in the manual. That picture makes it much more obvious, I see now how it concertinas around the edge, a bit like in a caliper.

Thanks for clearing that up. Another one for the V40 FAQ & Cookbook. And if it were possible to search for "FAQ" I'd be glad to compile one, but alas, strings of less than four characters can't be searched. Anyway, thank you.

[ITSv40]

Originally Posted by canis

Ah, right. Gotcha. They should explain this stuff in the manual. That picture makes it much more obvious, I see now how it concertinas around the edge, a bit like in a caliper.

Thanks for clearing that up. Another one for the V40 FAQ & Cookbook. And if it were possible to search for "FAQ" I'd be glad to compile one, but alas, strings of less than four characters can't be searched. Anyway, thank you.

No worries, it took me some time to realise why the diaphragm was always out of the cap when I first had a V40. Hope you get your petrol pipes sorted, sounds a right PITA.

[canis]

Originally Posted by ITSv40

No worries, it took me some time to realise why the diaphragm was always out of the cap when I first had a V40. Hope you get your petrol pipes sorted, sounds a right PITA.

Took a day off from it today, sorted some rust on the front panel instead. It was raining this morning, so didn't get much done. Need some decent jubilee clips.

[pierremcalpine]

Originally Posted by canis

Still a bit confused. You press in the diaphragm, and go to put it back on the reservoir, it pops out again just in time for you to reach the fluid where it hits the surface and displaces a load all over the sides. I'm not getting this, am I?

If the rubber thingy is installed snugly in the cap itself, when pushed inwards it should stay put without issue.