Membran

If this part has ruptured or degraded through hydraulic fluid damage, replacement is the sensible thing to do, particularly when it costs so little to ensure the fluid stays within the reservoir vacuum and free of air bubbles.

For the rest of your brake components and consumables, take a look at the online 'parts list' fiche for your model, and specify a multiple order for all the parts you need to complete the job. Ultimately saving money on shipping costs, and time in your garage, when you find you overlooked a small but essential component!