I've rebuilt and replaced a number of the clutch master cylinders over my 28 year ownership.
It's really easy to rebuild one of these with the overhaul kit and a lot cheaper. I've found that it's the rubber seal that eventually degrades at the top edge. That's probably where it's most susceptible to water and air ingression and eventually deteriorates. The bore wasn't corroded at all. So long as the new seal is treated carefully during the rebuild it should last several years minimum. The kits usually supply some red type grease that's needed for first fitment.
http://www.rimmerbros.co.uk/Item--i-GRK1029
IIRC the circlip is a bit fiddly to remove and replace but apart from that if you can replace one master cylinder for another then it's a n easy job.
Hope this helps
Adam
TR7 V8 DHC Jaguar Solent Blue. 9.35cr Range Rover V8, Holley 390cfm, JWR Dual Port, 214 Cam, Lumention, Tubular Manifolds, S/S Single Pipe Exh, 3.08 Rear, 200lb Spax & PolyBushes all round, Anti- Dive, Strut-Top Roller Bearings, Capri Vented Discs & Calipers, Braided Hoses, 4 Speed Rear Cylinders, Uprated Master Cylinder & Servo, AT 14" 5 Spokes or Maestro Turbo 15" Alloys, Cruise Lights, S/S Heater Pipes, Replacement Fuel Tank. No Door Stickers. Mine since July 1986, V8 from 1991 courtesy of S&S V8 conversion kit (built not bought) and big brake kit.