I put the cache operator underneath the render operator in the global portion of the pflow. I set my ranges, and set to manual update. I set my memory limit to 1Gb.
After manually updating, the memory usage for all frames remains at 0, and there is no sign of any caching; pflow still updates.
There's only about 2k particles, so I wouldnt have expected the cache operator to have any problems. I am using a birth material (box1) if that has any influence on the situation.