This is the K100 that has stood for years.  Yesterday I reassembled and bled the rear brake.  The bleeding began by gently pumping the pedal and watching the bubbles rise in the reservoir.

I then used a bleeder that consists of a short hose with a split in the end - this lets fluid out but doesn't let air back in.  This all went very easily and the brake appears to work fine.  I can't try it on the road as yet.  

But when using the brake bleeder the caliper pistons were sucked back away from the disc.  The question is, could this mean that the master cylinder return hole is blocked?  Do the bubbles rising in the reservoir come from the feed or return hole?

I see from an earlier thread on rear brakes that I can't remove the plastic feed nozzle to the master cylinder without destroying it.

I suspect your brake bleeder is creating the odd behaviour. Two easy ways to check:

1. Attach a normal piece of hose and check it that way. If the hose is transparent/translucent put a big curve on it to allow the fluid to rise up before dropping into the container. That way you can see the bubbles.
2. Leave it over night and check the brake tomorrow. If it's still good then yes, the bleeder (!! ) is the source of the behaviour.

Thanks for that.  I really like the sound of number 2 because I've already left it overnight and it's fine 

Strange thing is, I've been using this brake bleeder for years, and only recently on the front brake.