Brake problem with my classic mini

[Cooper1999]

I'm hoping someone can help with a problem I can't fathom out.
My mini started pulling to the left under braking recently.
Now I've had this problem before, caused by the pistons in the caliper seizing. Last year I ended up changing both calipers (using parts set aside for the MM), and while I use the car throughout the winter, I've never had them start to seize this quickly.
When I swapped over the wheels (back to summer tyres) the RH disc did look as if it wasn't being worked - it didn't look corroded, but wasn't the normal bright silver that the LH disc is.
But on pushing the pads back, both piston appear to move freely. I did this a few times (pads back, then press the foot pedal) and it did seem to make a slight improvement (but this may have been in my head!) then back to how it was.
To the extent that, on a clear straight road the car still pulls to the left and I can lock the left wheel but not the right (at the point of lock up).
I haven't tried pushing the pads back on the left. on the assumption that this side appears to be working normally.
Any ideas what the problem could be?
Tyre pressures checked, as have been the wheel bearings, ball joint etc. No brake judder either (disc seem to be true).
Thanks,

[Cooper1999]

Okay, so I've tried bleeding the RH caliper this afternoon.
No sign of air in the system, and no change in the drive.
This photo attempts to show the state of the disc when I removed the wheel:

The hoses, while a good few years old, are Goodrich braided jobs.

[Cooper1999]

Thanks for the responses and suggestions.
I'll be looking at this again this weekend.
To answer one of the above points, the rear of the disc did look to be in a better state than the outer face (as seen in the photo).
I did wonder if the problem could be master cylinder.
More detective work required.