A disc for use in a pneumatic valve includes a stiffening insert encased in a flexible body. The stiffening insert acts to protect a central actuation region of the disc against radial compression, which may arise if the disc is used to seal against a bore hole, and against material flow towards the actuation region away from a clamped periphery. The behavior of the disc in a valve assembly is therefore more predictable and reproducible. The flexible body may further be tensioned across the stiffening insert.