Are your water pickups on the front lower part of the drive? It is known that Shorties when trimmed positive will lose water pressure because of the Negative pressure of the water flowing over the front of the bullet with the bullet pionted down. I believe you have to dremel out the sharp edges of the water passage to get water to flow into the hole in this situation.
also it seems like your bypass's are working fine. some Merc's reqiure 50ish psi. thats where the sceniero above showed it's Head..