Gave up one shimanos after the seals failed on my XTs one winter, apparently there are some model years which can’t hack cold conditions. I was tempted to go for new ones but apparently some of the newer models also can’t hack the cold? And as far as I’m aware, NO, spare seals are not available.
Last hopes I had were pretty woeful, very little power and lots of fade. Quite prepared to accept they were dodgy pads or something, but the XTs I replaced them with were much much better (until the seals failed, see above).
BB7s are lovely, thats what I’m running now, but you do have to be prepared to USE the adjusters on the trail, I was slightly taken aback by how quickly the rear brake lever came back to the bar on a relatively straightforward but wet descent. Also, the stock pads are pretty screechy, if that sort of thing bothers you.
next hydros I buy will be the Magura MT range, they promise the earth in terms of stiction free pistons, squeek free performance and of course the leak free warranty. I guess the MT4 or MT6 would be your perfect brake, the MT2 looks fine but is neither light nor bling.