Hi Adrian,
I started "this thread" last year with my CB400F2, which had exactly the same symptoms. New pads, new (OEM) hoses... and loadsa good advice from this forum.... all to no avail. I was extremely suspicious coz one old Honda (750K2) had a lever you could barely move, whilst the 400 lever would come right back to the bars. Lost count of how many fresh litres of DOT # went through it's pipes.
Anyways, ordered a DSS master cylinder servicing kit, and, well Bob's your uncle. New seals etc, & 35 yrs of crud & varnish cleaned out of the m/c. All fixed now. Don't especially understand why... there was never any leaks. Simply, it was the only thing I hadn't tried.
If you go this route be prepared for a bloody nasty little circlip which will do its best to defeat you... tip of the day would be to get decent long nosed circlip pliers, and budget an afternoon...
Good luck,
Simon