If a vp shunt reservoir is slow to refill, does that mean there's not a lot of fluid to drain or that it's getting clogged?

You are right: Some of the shunt valves, but not all, have one-way valve. Newer ones have occlusive mechanism on both distal and proximal sides of valve/reservoir, so that we can test the problem. It is probably best for you to ask your surgeon examine, if you have question. I favor my patients not to press the valve repetitively. Skin over the valve might weaken.

VP reservoir: A slow to drain vp reservoir may mean many things. It may mean the valve is in brain tissure, the ventricle may be over drained, there may be a blockage. Patients should not pump there reservoir.