By coincidence, I just got my Flavia back yesterday from it's slave cylinder overhaul. New seal kit from Omicron ~£66, plus a new diaphragm for the vacuum chamber. Required several rebuilds as the brakes kept locking on. Lots of greasing up of the compnents and the new diaphragm eventually fixed all of this.
The pedal still has a long travel (about 50%) before it starts to firm up, but the bite and brake feel is good once the travel is taken up. Giving the pedal a second pump brings it up to where I feel it should really be biting in comparison with driving a modern car. The pedal has been like this from the start so nothing to do with the rebuild. Brakes have been bled thoroughly so I don’t think it is air in the system as it only takes one pump to get the pedal properly firm.
I thought teh extra travel might be “knock back” due to wear in the wheel bearings or warped discs pushing the pistons back, but when static and the engine off, you can pump the pedal twice to get it high and firm, wait a few seconds and the travel is again about 50% before going firm. I guess that could be air in the system, but I’ve read that stainless steel lined pistons tend to allow the pistons to glide further and more easily back into the calliper, where as pistons in unmodified calipers will stick and retract less after being released hence the pedal is higher.
Of course, it might just mean that I need to fit new pads!