You can check for a cracked lid by merely pouring water over it and see if the air bubble in the pump goes away. It will suck in water instead of air.
Sure hope you don't have to dig up any pipes to find the crack if it is elsewhere. But you could probably saturate the soil around the pipe using the same above mentioned technique to see if it stops sucking air when a section of the pipe is covered with water.
Good luck!
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