After driving the car for like 30mins non-stop and then i parked the car for 5-10mins.If i want to re-start the car again,the brake will be flat.If i pump the brake, it will come up a bit and after driving for a while it will return to normal level.This might not happen again for another 3wks or thereabout.I've taken the car to the mechanic for him to check the brake fluid and the pad but he said they were OK(i did not stay with him so i can not confirm if he really worked on it).Pls house,advice me on what to do.