These are to be used in conjunction with the brake bleeder tool. The tool comes with a 10mm. You will need to find out the thread size of your Master Cylinder screw to determine which one you need. We do know a few of the chassis.
CRG Ven99 Brakes – 6mm
Birel 2006 and up typically use the 10mm Fine Thread
Some TonyKarts use an 8mm.
SKM, Righetti Ridolfi, Intrepid, Topkart typically use the standard 10mm that comes on the bleeder.
These are just a few, but the only way to be 100% is to measure yours. Especially if it is used, you never know if someone replaced the OEM master cylinder.
To prevent damage, only screw one thread of Birel and OTK brake fittings.