Advice needed for Stuart Turner Water Pump

I have an AF2E12 'Boiler Feed' water pump that has until recently worked without any problems.

Recently I had noticed a drop off in water pressure when the pump was in use.
I opened up the pump to see what was happening. This pump has rubber 'O' rings on the piston so I renewed these I also checked the four brass valves and their associated springs. The valves were making a good seal in their seats and the stainless springs were in good condition.

I renewed the gland packing around the piston rod, made and installed new gaskets all round.
Once the pump was re-built I filled the reservoir with water and turned the pulley as fast as I could by hand (as I had done many times before) and water began to be pumped around the system but after continuing to turn the pulley for a while the flow of water slowed to a dribble and then stopped. Now, no matter how fast the pulley is turned the water will not flow.
I believe these pumps are self priming and up to now I have never had to prime this pump before use (I have always emptied the pump of water after every rally)

What could be causing this recent loss of pressure and why has this pump stopped pumping?

Below is a short video of the pump working.

Pump working

Tried flushing the pump, muck maybe lifting a valve off its seat.

Maryalice