Yes, it's a diaphragm to allow fluid level changes such as pad wear/replacement etc. And also seal against fluid escaping from the cap and it helps to keep moisture out.As far as I understand - its there as a seal and also to allow expansion/contraction of the fluid.
Can someone tell me what's the point of the insert in the front brake reservoir, please?
Is it there to save on fluid ???
Ta
As far as I understand - its there as a seal and also to allow expansion/contraction of the fluid.
Yes, it's a diaphragm to allow fluid level changes such as pad wear/replacement etc. And also seal against fluid escaping from the cap and it helps to keep moisture out.
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I don't know about the sponge but I always thought (assumed) the bellows/diaphragm thing was to fill the air gap between oil and cap to prevent splashing and bubbles forming which could then get pushed into the hydraulics. The bellows/diaphragm gets "sucked" down as the oil level drops as the brake pads wear so it's always sitting on top of the oil.
I assume the sponge does something similar. Perhaps any bubbles cling to the sponge rather than dispersing through the oil. Dunno. Making it up now.
Stop. You're all talking about something completely different.
Later reservoirs fitted with the sponge insert still have the bellows/diaphragm fitted as per normal.