Cheers @Fozzie
Yes, tried all the above, I'm not actually losing fluid, although it would be difficult to tell as it would be a tiny amount.
Reason for the caliper rebuild was poor braking, and having to top up twice in 400 miles.
I've tried the cable tie trick a few times, it works well, but the pressure ebbs away quite quickly.
I'm going to pull the calipers off to check for leaks around the piston seals, as all the banjos are dry, but I'll leave the brake lines conected. Whilst doing that, I'm going wind the calipers back and open the bleed valve, as there could be air trapped behind the pistons.
Thanks for the help.