Sometimes when you put on the brake pads the fluid is backed up in to the resevoir as a result of compressing the caliper. This pushes the fluid back in to the master cylinder or resevoir. So. Look at the fluid to see if it is overfilled. Secondly, I would crack the bleeders on the system to see if there is any air in the system. Next, if that doesn't take care of things I would examine the master cylinder. It could be weak.