My Series 2 Fulvia 3 had 8mm A/F Bleed Nipples, I sent the calipers for rebuild and the company did an excellent job but they did come back with 7mm A/F ones! You would be hard pushed to go any smaller as the thread is M7x1.0
The caliper is a refurb from Omicron and as above my 8mm spanner is way too big. Thanks Simon, will try 7mm first.