Originally Posted by dirtbikedude

It just takes a small bubble to cause problems. If the pads are in good shape AND it has been happening since you first bought the brakes (are you the original owner?) then there is a good chance they need to bleed. Have that done and if it still persists then there cound be a problem with the seals or the connection between the lines and caliper/master cylinder.

Do you notice any fluid on the hoses, caliper or rotor? If not then just bleed 'em

Most systems are bleed properly but you do get one with air in the lines from time to time.

DBD