Liner in Master Cylinder Resevoir

[Berger21]

Does anyone know what the purpose of this liner in the resevoir is? It is black and rubbery and does not have a hole in it for the fluid to pass through to the MC.

[DaddySS]

It's a seal that minimizes the amount of air in the resevoir to allow the fuid to move in and out of the master without drawing air in. so the air fills behind the seal as necessary without mixing with your fluid.

You've probably seen the same principle in operation with the baby bottles where the formula goes in bag and the bag contracts as the fuid leaves the bottle.

[Berger21]

Thank you.