Common ram pump issues

Hi folks,

Built a ram pump a couple years ago. Worked ok although needed restarting occasionally. Had a storm after that season and only recently managed to cut my way through to the stream. I unhooked the pump, it was full of grit and silt so cleaned the whole thing out and reconnected. Made a new filter and it started straight away. Ran for 3-4 days and has stopped. Now won't restart. I press the gate/check valve and it claps 3-4 times, each time faster then stops. I'm kinda hoping that symptom points to an obvious cause. If it helps I'll go make a video and upload.

Air leak in the chamber?