Suction line will come From the hydraulic holding tank, lower lines....
The Only way you will continuously get air into the system is on the suction side of the pump... since you have changed the pump... that's the line to check first.
BTW... DO Not continue to operate the pump with air entering the system, it Will damage the pump and the drive motor will also not be lubricated with air in the oil...:smile:KennyV