My new Explorer pump was the same way when I first fired mine up, (Hydroboost brakes). It took quite a while to bleed all the air out of the system. After the first day I thought I was going to need a more substantial pump, but after driving it around for a week or so, it must have settled, because now it's has power at idle. It takes time for the fluid to rid itself of the air. If you turn it lock-to-lock to bleed it, the pump will cavitate and just makes it worse. Good luck. That three inch pipe is huge, looks like the exhaust guy did a nice job.
Mark