It isn't making sense. Seeing is believing, but I'm not seeing. Air has to come from the pump. If the pump is pushing fluid, everything is under pressure, including the return lines when they're returning. The HB unit can't "pull in" air. It can only push excess flow out the return, or to the PS gearbox. If it's foaming, it's coming from the pump, and it takes very little air to make a lot of foam. If there's air in the system, it will exit the HB unit return first. The PS gearbox return is much lower, and it will take much more volume before any air will show there. The pump is pumping air, from somewhere. Most likely a defective pump, with no evidence of a leak, anywhere.