is there a ball in the sump assembly?

yes
I thought it was a back siphon device to prevent dirty water from the sink entering the drum, as the sink water enters, the ball rises and seals off the hole into the drum.
Another version of this that you may find is a flap fitted on the pump outlet, as the machine pumps out the flap rises allowing water out, no pump action flap seals the hole .

wirecutterman
March 2010

some models do have detergent recovery balls built in to the bottom hoses

ck
March 2010