Brain - I think that Alstair did a good job explaining in post #13 of Bodgit2's thread from when he encountered problems.
Serious problem after replacing IP seals w210
I do not believe this is strictly a 606 issue. The valve in the 606 may sit higher in the IP and therefore be more exposed and prone to moving around than in previous pumps. The 606 IP is a milled aluminum body which may also come into play if over torqued or if torqued down with the valve misaligned.
Steve (Bodgit2) may have additional insights.