As I am not an expert I am not doubting you but what makes you say this?
1. Brakes not working
2. Caliper seen to be operating incorrectly
3. Caliper should be serviced/repaired/replaced
I am not sure I can go into any other detail!?
Brakes work like this:
1. Squeeze lever
2. Lever activates piston that displaces brake fluid in reservoir.
3. Fluid travels down hose to caliper
4. Fluid displacement at caliper causes caliper to squeeze brake pads
<--- fail here5. Bike slows down
As the fluid is obviously getting to the caliper, and it is just not operating correctly, it is reasonable to assume that this is where your problem lies.
Any idea how much a rebuild would cost - second hand maybe someone elses problem
Assuming the pistons aren't badly corroded (as this is uncommon) then it will just need new rubber seals which cost about $10, plus 1-2 hours labour to remove, rebuld & refit the caliper and bleed the brakes, and about $10 for brake fluid. If your brake pads are worn unevenly (probably) you will need new ones. They are about $30.
$10 + (2x$60) + $10 = $140.
If you have twin rotors on the front (not 100% sure if this was available on the ZZR250, it was definitely available on the GPX250) then they should both be done at the same time, which will increase the cost.