Wa Alaikum Salam.
I think your mechanic has messed up something with the braking system, or its relevant sensors, while changing the brakes pads. There seems nothing wrong with the brakes oil reservoir bottle.
IMO, you need to visit some good mechanic who can understand what has been done, and how to troubleshoot it, and 1st, try to take your car to the same person, and ask him to undo the mess created due to his ignorance and negligence.