it sounds like the check valve is bad, stuck in the open position. The check value is usually screwed into the tank, it may look like a brass adapter fitting. The small ones are not that expensive or hard to replace.

Looks like a 'Little Giant' brand. based on the picture there is a bunch of muck, so it is possible the opening that allows fluid to supply liquid pressure to the diaphragm. If it is plugged in, that could be the problem. Also, the discharge looks undersized too. this makes the pump work harder and has less performance (volume) . Replacement diaphrams can be bought if it torn. In the end replacing the pump is the recommended fix. I sold them for years.