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