master cylinder cover gasket sucked in

[oldspowered67C10]

Those "accordions" are there so that as the pads wear and the level of fluid in the master drops they can take up the space without letting out side air in. If youre finding that youre losing alot of fluid quickly then theres a leak somewhere, if you cant find any leaks on the outside remove the master from the booster to make sure its not leaking into the booster.