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Perhaps it just needs to be bled after replacing? Lift the tyres off the ground a bit, start the car and turn the wheel all the way to the left and vice versa to remove air. See that there's enough fluid in reservoir.
Check the clamp on the return line at the upper part of the pump. I had a similar problem when I changed my pump. That clamp was weakened and was allowing air to get into the system. A simple hose clamp fixed the issue.
Good luck!