Problem is that brake fluid is 'hygroscopic' meaning that it absorbs water from the atmosphere and because the rubber seals on the pistons in the brake slave cylinders are not -perfect- seals this then results in -some- fluid escaping past them and setting up rust occuring on the -outer- side of the pistons. Then when new brake shoes are fitted the pistons are compressed backwards into their sleeves along with the rust-pits residing on that part of them that was previously on the outside of the piston seal. End result....?? Leaking brake slave cylinders....!
Back in the 'good old days' when dinosaurs, and I, roamed the Earth, and -stuff- was expensive and hard to come by, you used to be able to re-condition/re-sleeve them, but now, when -stuff- is cheap, and (talented) labour is expensive and hard to come by, it's far cheaper to just throw them away and replace them with new ones from China. ....
Quote: "Only two things are infinite: the Universe and human stupidity; and I really am not certain about the Universe !" - Albert Einstein (1879 - 1955)