When you open the brake fluid reservoir on your peugeot 307 hdi the plastic unit seems to be sealed why?

The brake fluid reservoir on a Peugeot 307 HDi is designed to be sealed to prevent contamination and moisture ingress, which can compromise the fluid's effectiveness. The seal also helps maintain the proper pressure within the braking system, ensuring optimal performance. Always ensure the reservoir is closed properly after checking or topping up the fluid to maintain system integrity.