Brake Master Unit replacement for M10

Hi all,
Sorry for the duplication of this post: I have initially posted this call in the general Ercoupe Tech forum but I am now convinced that the part I am looking for is M10 specific and I hope I will find here folks who have possibly encountered the same issue before.

My mechanic cannot find any replacement to the Brake Master Unit P/N SBC 9201-1 made by "Gerder Product", see pictures attached. Univair has not. The mechanic has replaced the seal but it seems that the problem comes form the inside of the cylinder whose rubber coating is damaged.
Do you know where elsewhere we could find this item? Did you face this problem in the past and find a workaround?
Thanks a lot for your help!
Laurent