The valves in the pump have gone. They could be stuck open due to muck in which case you could clean them up and get it working or they could be broken, in which case I wouldn't bother with it. Some use ceramic valves, which are prone to breaking, but some use metal discs. if there is a noticeable increase in flow when you run it then it sounds like the piston is OK. Valves are nearly always the issue, especially in hard water areas.
I would at least dismantle it for a once over, nothing to lose there, and see what has gone. Hopefully you can access the valves without touching anything else.