My D19 was doing the same thing 6 years ago. We took booth Brakes apart and put all new in both side's. Ball's everything. 4 years later it startis again on the right side. I assumed it was from lack of use. So I bought a can of brake cleaner and squirted it in the hole on the housing in front where the spring hook's on. That seemed to help for a while, then if we would get real damp weather it would start again, more spray.
Finally we took the brake apart and it was just built up with dirt and rust. We cleaned everyting and it has worked like it should for the past two month's. Next time I will take it apart and see what the problem is.