The gasket just sets down in the recess in the clear cap. I have seen these caps with hairline fractures that are hard to see but will allow the pump to suck air in. Try some tape around it for troubleshooting. Also make sure that the strainer screen mesh is cleaned very well, clear lid screwed down snug with O-ring in place and that all fittings on the suction side of the pump are tight. When I have had a lot of air I found it simpler to hook to city water to fill all the lines (and purge of air by running all devices), hot water heater and fresh tank. Then unhook from city and try the pump. The fact the pump runs is a good sign. If you go over the system you’ll probably find something minor. In troubleshooting air/continuous pump operation on these systems on various trailers I have found different causes - a hairline crack in the strainer lid, a loose fitting on the suction side of the pump, improperly adjusted pump pressure/bypass settings, winterizing valve that was passing and allowing air into the system. Your pump settings are not likely the issue since your pump had been working fine.
This video by Reace might also help if you haven’t seen it