Check valves are a one way street sucking air, or in this case ozone, into the water system by venturri action usually coming into the lower footwell where you see many mini bubbles rising to the surface.
When installing a check valve blow into it from both directions. Only one direction should allow you to blow through it and is marked by a "flow" arrow. The other direction should not allow any air, or water from going back into the ozonator. IF you can blow through it both ways, it is a BAD check valve.
A bad check valve is possible, but two?
Now this is where I ask, are you familiar with the ozone inlet in the footwell, to which the homeowner says yes. Have you ever felt the heat coming out of the same inlet? Some say yes. Well, some restrict the water from from coming into the footwell as they investigate with their toes. Block the entry and you "force" the water presssure to build up and forces the water back through the check valve in the ozone line and into the ozonator. This is a posibillity.
When installing your ozone line that attaches to the mazzie injector to which your check valve line is attached to should be "looped" to go above the water level. This prevents the natural back flow IF the check valve fails.
