Power Steering Overflows

I recently rebuilt my leaking p/s pump on '74 Valiant /6. Replaced rings and seals and then reinstalled with a new low pressure hose. I did not drain all the remaining fluid from the steering box only added the proper amount of new fluid to fill the reservoir. Carefully bled the system by raising front end and turning steering wheel lock to lock 20x, then another 20x, then another 20x to remove air. No leaks!

But now with the engine running and the cap removed the reservoir boils up and overflows fluid only when I turn the steering wheel (front end still raised). Noticed dirty fluid mixing with with the new clean fluid so will do a complete flush but what would cause the overflow?

Power Steering Pump.JPG