I was going to suggest that desireedog2 generate a hydraulic schematic but you did this verbally. I have dealt with hydraulic systems but never a dump bed specifically. I was wondering if this system may have separate safety hold valves or ones that would be build into other devices in the system. That way the bed could not go down (or maybe even up) without a specific hydraulic signal.

A malfunctioning hold valve could cause confusing up/down problems.

An accurate hydraulic schematic is always the first step of diagnosing an odd hydraulic issue.