These guys should be able to sort you out for rebuild kits for your master cylinders and calipers. A MK2 Metro MC rebuild kit is £20 a rebuild kit for the AP 4 pot calipers is £20.
Bigg Red Ltd
The Workshop
Weir Lane
Worcester
WR2 4AY
Tel: 01905 428793
Fax: 01905 427035
Email: calipers@biggred.co.uk
web site: http://www.biggred.co.uk
Just used them for my rebuild parts, very good service.
Rich