mysterious dump bed

I have a 1997 cab chassis Chevy 4x4. The dumping flatbed is made by Knapheide, it is 8' x 9'. The pump is electric, power up an down.

When I lower the bed, it goes down smoothly and lays flat on the platform frame. The problem is, it lifts back up about 1" by itself after a few minutes. It does not matter if the truck is parked or driving the result is the same.

Has anyone experinced this before and how can I resolve this problem.

Thanks Scott

 

Is their a posiablity of air in the system?. does it occure when their is a load? or too much fluid?