Fully rebuilt the calipers last winter, new seals, pistons, pads, pins and hoses.

All working fine but i have had 3 instances in the last 6 months where, after a spirited bit of riding i'll stop somewhere for 20 seconds or so with the brakes on (eg a junction or set of traffic lights) then when i pull off and get to the next braking point the lever has gone dead on me. A few pumps of the lever and its all back again fine but it has pull the wllies up me a couple of times.

Any idea whats causing it?

Did you rebuild your master cylinder too?

All I can think of is that when you keep your brake on, the pressure forces fluid back up into the reservoir (which it shouldn't)

Or maybe a tiny amount of air in your system? Like miniscule.

Sounds like they havent been bleed properly

What he said. It's a pain in the backside, but it wouldn't hurt to give them another bleed to see if there's anything lurking.

I rebuilt the m/c about 3 months befoe doing the full caliper strip/rebuild. so pretty confident it's good.

I've bled the brakes about 5 times (i do it fairly regularly as i cant stop myself tinkering!)

Maybe a mitivac bleed is worth a go....

You say spirited riding, could be heat in the discs warping them slightly and pushing pistons back into calipers, had this problem on my TL1000R turned out to be a warped disc