I haven't seen a shellback pump, but if it's like most pumps, you will need a check valve to prevent pumped water from dropping back into the tub.
A pump without the checkvalve will cycle all day.
Dear sirs, could you please instruct me on how to adjust the settings on this pump.Currently it is shutting on and off continuously.We have a washing machine, and our water softener draining into this tub, I think at different times.I know there are two adjustments, a pipe with a hose connected, which slides in and out, and a threaded fitting, with a locknut, which screws in and out. Any help would be greatly appreciated. Thank you.