A solution of muriatic acid pumped into the line will take care of built up scale. You want to pour it in the bowl, then flush it out of the mechanism with normal flushing for a few strokes. Leave in the line for about an hour.
After that, a similar monthly treatment with vinegar will keep things tip top. I always follow up the vinegar with some mineral oil to keep the pump mechanism seals lubricated and happy. Beats the alternative!