Hi,
Someone else here gave me a suggestion that worked really well with my clutch slave cylinder, should work on any of the hydraulic masters or slaves on the car. Use a grease gun.
First, fit a zerk where the hose usually goes. Leave the bleeder screw tight and plug any other holes (a bolt with the right thread will work temporarily). You may need to take the cylinder with you to an auto parts store to find the right size zerk. My TR4 has fittings throughout that take 3/8" zerks. Larger zerks like this are often tapered, sort of a fine pipe thread I think, so won't screw very far into the straight threaded fitting off the cylinder. Still, it may be enough to temporarily fit one and do the job.
If however the grease comes back out around the threads of the fitting and the piston still doesn't pop out, consider using a die to cut and straighten the threads on the zerk so that it fits the cylinder better. In my case, a standard 3/8" NF die worked. I think some later TR6 and/or replacement parts might have metric threaded fittings in some or all of the hydraulics.
I agree that it's possible the cylinder is damaged by corrosion, if it's so badly stuck. If so, you have a choice to replace it or to have it sleeved by someone like Apple Hydraulics.
Here's another tip, whatever you do to get the master cylinder back into usable condition, use a proper lubricant when reassembling it. It's possible to just use brake fluid for this purpose if a car is immediately being put back on the road.
But, there are better greases and oils made for this sort of reassembly that should be used instead, most especially if the car will be sitting a while during restoration. If brake fluid is used and/or the rebuilt system is filled with brake fluid now, and the car sits for a few months, new corrosion will likely set in and you could well find yourself doing the job all over again in 6 months or a year.
Look at auto parts stores for greases and oils that are compatible with brake fluid, made specifically for this purpose. I know Raybestos makes an oil, and Lucas sometimes supplies packets of grease in rebuild kits. Use just a little on the seals and all rubber parts and wipe a very light coating on all metal surfaces inside and out, to keep corrosion away until the car is ready to use.
When it's time to take it on the road, fill the hydraulic systems with fresh brake fluid, bleed them and you'll be ready to go.
Cheers!
Alan