Mercury Racing K-Plane slow to go up
 

Have a '99 Prowler. Portside kp goes down at normal speed but is very slow coming up. Pump loads as it should going down but appears to loose suction when going up. Motor speed is higher. Is there an internal valve I need to check or replace. Before I started taking it apart I thought I would ask for some wisdom first. New fluid (royal purple synthetic ATF) last year. Any suggestions would be appreciated.

Ed