Subject: Re: Master cylinders...
Christian, the cheapest spare part I've ever fitted to my car was a £1 plastic brake fluid reservoir cap, 25+ years ago! Bought in a scrap yard and worked perfectly ever since. What is the "corroded aluminium" (or whatever) component inboard of the master cylinders in your photo, which I don't recognise?
PS you only have to look at the brake pipe runs to see that "bodger" has had a go at your car in the past