Bump!
The outcome of this was that when I finally got the MC apart, there was a small amount of pitting at the low point of the bore, and also around the circumference of the rear seal, which is where it was leaking from. Despite trying to remove it with some 1000 grade wet and dry paper, it was too deep and the pitting, especially around the rear seal, remained.
In the end, and with Lotus in the Peak approaching, I bit the bullet and bought a reconditioned MC from Lotusbits. Not cheap, but very well finished with a silver anodised coating, and supplied with a clean (inside and out) reservoir, which was a nice surprise. The cylinder was easy to fit, and did not require the clearance from the servo pushrod to be adjusted (still about 0.5mm). I did, however, learn a thing or two about bleeding brake systems. Unfortunately, needing two hands for most of these operations and with brake fluid everywhere, I didn't get any photos...:
1. It is important to fill and bleed the MC itself before connecting it to the car. After watching some Youtube videos, I bought a MC bleeding kit from ebay, make sure the one you buy has 2 x M10x1 plastic plugs for the ports on the MC. This was for the Bendix MC, not sure if the connections on the Girling are the same. I connected two clear tubes to these and fed them back into the reservoir. I gently operated the brake pedal (only over its normal range of movement), and watched the bubbles come down the tube and disperse back into the reservoir. After a while the bubbles got less and eventually stopped, with just a smooth flow of fluid. This happened quite quickly for the front brake port, but took a lot more pumping for the rear brake port before the bubbles stopped. At this point I unscrewed the plastic plugs, and quickly refitted the car brake pipe metal connectors. Luckily spillage of brake fluid was minimal.
2. Once the system was complete again, I bled it using a Gunson Easy Bleed. £20 or so from Halfords and worth every penny. The Easy Bleed applies air pressure from a spare tyre to the reservoir, forcing fluid through the system and out of the bleed nipples. Much easier than pumping the pedal, and there is no danger of damaging the cylinder seals. I bled about 600ml of fluid through the system, but found that I didn't have any bubbles at all, which was good.
A test drive showed that the brakes felt firm and responsive, with the pedal travel and load being much the same as before. But having had the rear calipers rebuilt a couple of years ago, and the front calipers replaced recently, the braking felt much more assured, with none of the pulling to one side that was symptomatic of the old calipers being past their best. All in all, a satisfying result.
Enough to take some of the paint off the servo.
