Thick lube like vasalene libarely applied is the only answer I know. Believe the valve is like the LS-3a which uses a plastic ball on a stick. Goop lube on the ball as the rod is slowly turned. Do this before every flight where water is carried. May be able to reach the ball hith your finger, if not use a swab on a bent wire (coat hanger). Make sure the dump lever on the root rib (fuselage side) has a enough slop so that it isn't shoving on the rod before you want it to shove on the rod during dump.
Hope this helps,
JJ