I installed a backup sump pump following the instructions. The backup pump will turn on but does not eject any water from the pit, the water only swirls around. The pipes aren't clogged: the main pump works great and it is new pipe on the backup. I tried drilling a hole in the pipe above the backup and water will shoot out the hole when the pump turns on but not up and out through the main discharge pipe. There is a check valve on the main pipe, but is working as the main pump does empty the pit. My only guess is that the backup line hits the main line at a 45 degree angle and the water coming up doesn't have enough force to open the check valve. Again I installed it as indicated in the instructions. The backup is battery operated and both the pump and battery are brand new. Anyone have any ideas?
Letter I in that diagram, the primary pump check valve, is your problem. It is either missing or not operating correctly. The water being discharged by the backup pump is draining out the inlet of the primary pump.
Also, the primary pump check valve needs to located below the wye fitting and above the primary pump like it is in the diagram. If it is above the wye fitting, then it won't work.
I'm going to take a guess that your check valves are not in the right position. Both check valves should be located before (lower than) the 45 degree angle connection.Your new pump is pumping the water up to the 45 degree connection and then the water is running down the pipe to the old pump and into the pit causing the swirling that you see in the pit. Now if the check valve for the old pump is below that 45 degree connection the water will not be able to flow down the pipe and into the pit and will be forced up and out.