[Oldsguy]

You need the wire clasp. If the lid doesn't stay tight then the fluid will become contaminated from moisture in the air (or airborne dust, etc.). Brake fluid is hydroscopic (sp?) it has an affinity for water. That diapharm is a gasket and is shaped like an accrodion to take up the spare space in the resivoir as the fluid level drops down. I don't know if you can buy just that wire clasp or not but it is worth a try, otherwise you will have to buy a whole new master cylinder.