I assume you mean front master cylinder? If yes then it should be Magura on R850 and R1100s. At least my R1100GS 1998 has Magura and I've replaced the seal once - started to slowly leak in Jordania, but I just needed to top up the reservuar and breaks were fine and carried me home w/o any problem.
Piston+seal kit is not cheap, I paid around 50 EURos if I remember correctly, but you only need to replace it every 3-5+ years or so (I replaced mine first when the bike was 8 years old!). Keep the break oil fresh - if possible replace every year or at least every 2 years - this'll extend seals life considerably, since break oil gets VERY acid in time starting to eat details and seals in the system (this is why I'd rather pefer OEM part that's been given thought on than an aftermarket piece). Breaks (=security) are something where you shouldn't cut down the costs.
The BMW repair kit no. you need is: 32722332037
(No. 20 on the pic):
Try BMW Motorcycle Parts // Welcome
for cheaper prices on BMW bits if you can have 'em shipped anyway.
Also the reservoir seal (no. 12 on pic) is starting to wear out on mine (because opening-closing), so on older bikes I'd recommend replace reservoir seal (rubber boot in common language) as well (part no. 32722312421
Piston is D=20mm and with two anti-phased triangle-profiled seals on both ends if it's any help, but I guess it's very Magura-oriented stuff that's hard to find elsewhere, and even if you find I reckon it'll last considerably less than OEM part ("pay less get less"). I may be wrong on this tho.
Take care, Margus