I've been following this thread and at the risk of raising eyebrows, I would like to offer several observations:
I began using the D1/classic about 18 months ago and have yet to get any water during inhalation, regardless of the position. I frequently dive inverted and/or face up while performing inspections of marine structures. Several of my employees are also using the D1/classic during inspection dives. I have yet to hear of any complaints from them about wet breathing. Prior to purchasing the Hog regs, we were using SP and while I had no complaints with them, they were well worn from years of service. Considering the initial cost of the Hog, the availability of service parts and the positive reviews, it was clear that replacing the SP with the Hog was a good decision. After having logged more than 100 dives using the Hogs, I would stand by my decision.
As an engineer and a person who performs their own maintenance, I think the design of the D1 and the classic second stage is simple, reliable and well thought out (except for the turret). And although it may appear to be similar to Apex, it is not a clone. There are observable differences. One such difference is the seat for the exhaust diaphragm. If one were to observe any of the Apex and the Hog classic second stages side by side with the exhaust diaphragm (valve) removed, one could see the difference. That being said, the effectiveness of the seal between the diaphragm and the seat (plastic body) is dependent upon the pressure gradient during inhalation and the contact area between the two items (one flexible, the other rigid). In the case where a diver has just completed exhalation and the exhaust T is parallel/exposed to high current velocity or scooter prop wash, it is possible that the diaphragm could be lifted from the seat or held open. The effect is similar to air flowing over the leading edge of airplane wing.
Typically, the diaphragm is held seated by a combination of the hydrostatic gradient during inhalation and the compliancy of the diaphragm. There is a very short period of time immediately following exhalation where there is zero gradient pressure across the diaphragm. Since the compliancy of the diaphragm is extremely small, it is possible that water flowing at a substantial velocity over the exterior surface of the diaphragm could create sufficient lift to open it. The potential for this condition increases as the respiration rate increases since there is a shorter time between exhalation and inhalation. Shallow, rabid breathing would be the likely catalyst for experiencing water ingress past the exhaust diaphragm. Increasing the rigidity/reducing the compliancy of the diaphragm would tend to reduce this possibility but would result in a slightly higher WOB associated with exhalation. It should be noted that debris such as sediment, drift algae or macro plankton could become trapped between the diaphragm edge and the body, creating a leak.
Performing a static vacuum test will rule out debris trapped in the seat but would not provide any insight as to why the regulator would allow water to pass by the diaphragm while in high current, scootering or turning one’s head quickly. Covering the upstream side of the exhaust T would tend to reduce or eliminate the dynamic condition of the diaphragm being lifted by water flowing over it and could be used in a process of elimination for determining the leak.
BTW: the SP second stages we were using were the S600 (plastic barrel) and they would leak slightly (very infrequently) when inverted. We also use Cyklon 300 but these never leaked however they do not breath quite as well. The Cyklon were preferred on long dives however as they did not contribute to dry mouths nearly as bad as the S600 due to the metal body.
