Possibly failing flex hose. They fail internally causing less fluid flow. If they haven't been replaced in a several years, I would recommend replacing both left and right. Could also be failing caliper. Possibly bad master cylinder, but if only one side then hoses and/or calipers may be problem.. I have had these issues in the last few years with trucks that have had components that were very old. Hope this helps. Note...Always replace brake parts in pairs.