To try to fix the problem of burning brake fluid in the engine, I've started overhauling the brake system components.  As it's the easiest to get at, I've started with the master cylinder.  Following the instructions in the manual (and the paper that came with the overhaul kit), I've got as far as pulling the piston out of the body of the cylinder.  The instructions say it should just pull out, although to be aware that the o-ring could slip into the circlip ring and make it tight.  However, I can't budge whatever it is that's stopping the piston from coming out.

The bearing that guides the piston, jammed in the bore of the cylinder.

It appears to be the bearing that guides the piston that's basically jammed in the bore of the cylinder. It's designed as an interference fit, apparently with an o-ring part-way along it, so if there's any sort of contamination got into the cylinder (after 40+ years of use), that could cause it.  The piston itself moves freely within the bearing, but once it comes up against the "stop" at the open end, that's it - not even a little movement to suggest that it will come out.  I've tried pulling with padded (so as not to damage the surface of the piston) pliers, but still can't get anything.  I've also tried to lubricate the bearing with brake fluid, brake cleaner, and penetrating oil, but to no effect.

I've been told that the bearing often breaks eventually when trying to remove them from the brake cylinder, and that they're no longer available (it wasn't part of the repair kit that I had ordered).  So, even though I'd still like to know what the condition of the inside of the Master Cylinder is like, I've decided to buy a complete (Lockheed) master cylinder, as well as a complete servo assembly in the interests of time (and my sanity).  So all the parts have been left to one side and I'm currently waiting for the new components to arrive. (The delivery van actually called yesterday to drop them off, but it was during the only 15 minute period that I wasn't around to open the door, so now I'm waiting as long as I can before moving away from the front door, on the assumption that they'll re-try to deliver mid-afternoon today.  It's like being a kid again and waiting for the time to open birthday or Christmas presents).