Tony Moss had a similar problem around a year after fitting a new servo and new master cylinder. His local garage diagnosed that the servo wasn't working properly, so he got hold of a replacement and came to my workshop so that I could help him fit it. When we lifted the master cylinder off the servo unit, the reason that the servo wasn't working effectively became apparent - it was full of brake fluid
We came to the conclusion that the "new" master cylinder was new old stock that had been sat on a shelf for at least twelve years and possibly longer. The seals had probably become brittle during that long period of storage. My preference now is to fit new seals into master cylinders rather than replace the whole cylinder, as that way I get to see the condition of the seals. I now have a set of honing brushes that will clean up the inside of master cylinders to remove any rust, scores etc.
We took Tony's servo apart, which I must admit I hadn't done before and cleaned it all out. We lubricated the seals on the valve with red rubber grease and talced the diaphragm (stops it scuffing on the housing) then put it all back together again and I believe that it has been working fine ever since. I then learned that the service parts for the servo are the same as used on the later Minis, post 89 -
http://www.minispares.com/product/Class ... 0to%20shop.
Mini servo kit by
Alistair Courtney, on Flickr
The only difference being that the Mini kit comes with all the valve seals fitted to a new actuator rod, which has a screw thread, but the Metro actuator rod is shorter with a fixed clevis fork. With a bit of care and some red rubber grease the seals can all be transferred to the Metro actuator rod. The servo repair kit and master cylinder seals are now made by Caparo, who bought up AP a few years back. They list a Rover Metro/100 servo repair kit in their catalogue, but it is not a stock item and I haven't been able to get hold of it anywhere.