After the brake warning light went on, the braking system has been serviced. The mechanic found one of the rear brake cylinder leaking and replaced it. The braking system was checked on a special device that checks braking power of each wheel separately and found working perfectly.
However, the warning light won't go off. Can it be a combination valve problem? when i remove the connector from it, the light goes off.
Should it be replaced or can it be reset somehow?

Thanks.

Provided there are no leaks and brake shoes are adjusted correctly; switch could be reset by bleeding front brake -either side. The difference in hydraulic pressure causes a small piston in combination block to move it depresses contact for light. If piston gets stuck sometimes it can be freed by opening opposite brake system, the pressure in the rear will overcome the pressure in front and piston should move and light may reset. If it doesn't work, you will need new combination block.

Yeah I agree, you must have different pressurse in the two sides of your braking system. Hopefully bleeding should fix it.

Firstly check the brake-pedal switch, this is a small button behind the brake pedal is activated when you press down on the brake pedal and makes the brake light go on. if you look up you will see a switch with 2 wires on it the colors should be green with a red line unplug this and see if your lights go off if they do just replace the switch.