Why is power steering fluid spewing out of the cap after replacing bad leaking power steering pump

You have Air in the Power steering System and it needs to be bleed out, And to do this if you Jack the two front wheels off the ground and Let them Hang free so there is No Load on the steering at all. please put some type of stands under the car, You will need more power steering fluid but before you add this you need to start your Car and turn the steering Wheel Slow from left to Right a few times, You will hear a wearing Notice come from the pump and you will see more fluid come out of the pump as well, that is the air trying to bleed its self out. The fluid coming out will be air rated lots of bubbles in it, Now that you have done this the Noise from the pump should start to go quite at this Point you can start to Add the New power steering fluid slow to the reservoir and keep turning the steering wheel from left to right slow (Never Fast but slow ) Have a lot of rags to clean the Mess. Hope this helps you. :)