I remember years ago a couple of the local guys fitted hydraulic conversions to the rear drums to get round a similar issue.
Just doing a trawl on the internet and found this
Came from here http://www.accessnorton.com/arc-the-...t12396-15.html
Original Mini Parts are as cheap as chips so it wouldn't cost much to try it http://www.minisport.com/mini-spare-...o_21A1058.html
Dinky little Mini slave cylinder is less than $10