Dump slow to lower

The rear portion of that pump is the valve... It is built right into the pump itself.... I would start by checking out that the linkages between the cab controls and that valve are not worn causing excessive slack in the linkage. Excessive slack in the linkage could cause the valve to not fully move to the return position, which means it is only partially opening the return port inside that bypasses the pumping gears.

 

If that's the only line from the tank, then yep, that should be a control valve on the back of the pump. I've never seen a setup like that before. Pretty slick. Who makes it?

 

Ive seen simialar pumps before... I cant remember what the manufacture was, but I do remember that the rebuild kit for the one we had at the fertilizer plant was ridiculously expensive.

 

There is a linkage somewhere... The air just works an air actuator... Which could also have gone bad. If the air actuator is bad it might not be moving the valve as far as it should on the return stroke.

That actually is probably exactly whatbthe problem is... The air actuator is most likely bad, worn out, or something along those lines.

 

Do you lower the bed with the PTO in the on position???? If so try lowering the bed with the PTO turned off...

 

That actuator probably looks like a very tiny hydraulic ram... And is probably right on top of (or maybe built right into????) the pump/valve.

Actuators come in hundreds of sizes and styles but here is a picture of a more common design...


It is probably connected directly to that oval piece above the right side hose in the pic you posted of the pump.

 

Pump looks like industry standard 27 GPM dump pump.
The silver aluminum box shifts the spool valve. Have someone shift it and watch that spool is making full travel or take air shift cylinder off and manually shift it.