Is it possible that the pump is actually a Federal unit...not a TRW ??
I ask because I had no lower(2000 RPM and less) assist on my '68 GTX and it was the pump. There is a big spring inside the pump that keep everything seated in the rotor/valve part of the pump.....and it had broken.
I can't for the life of me think how you could have done something to the box by removing the valve unless a piece of old o-ring fell inside one of the ports.
Usually you MIGHT have to tap the valve a bit either way to center it so there won't be any "self steer", but you shouldn't lose pressure.