If you had connected the pump up incorrectly it would suck air in from the outlet and pump it into the tank. It works so you haven't connected it up incorrectly. On our pumps there is an arrow showing the direction of flow.
Is this pump and the pump you had before rated at the same delivery in liters per minute? If they aren't then that's the reason. For example, we have 2 Shurflo pumps on the van, 1 is a 3.8lpm pump and the other is a 5.2lpm pump.
The controllers are the same so I need to set the Varistream controlling that pump at 7 to have the same flow as the other pump at 4.
Controllers won't be the same either. New controllers could well have different internal components to older ones made by the same company. Electronic components could also be sourced from different suppliers. And of course, workmanship quality can differ as well - the reason why warranties cover faulty workmanship and/or materials.
Many years ago a friend bought 2 identical Siemens portable radios in South Africa, one for each of their children. If you listened to each radio on its own the sound would be totally acceptable. But when you compared the 2 together, the sound of 1 was much better than the other.
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