Without knowing anything about how the truck is settup I would guess the pressure relief is set a little to low or maybe the pump is getting weak. Empty the pressures are very low loaded obviously they are higher. Most of the time a weak pump will still make good flow at low pressure but as the pressure goes up the internal leakage does to and eventually the leakage is a significant amount of the pump capacity. It could be a filter although it’s pretty common to not have one and usually when they do it’s on the return side so it won’t affect the pumps performance but can make things slow to go back down
I’ve changed out pumps that have gotten slow lifting loaded dump boxes.
First was to make sure the air solenoid is shifting between up, neutral and down.
I’ve adjusted the pressure regulator for more pressure, no difference.
Next thing I did, turned the oil taps off. Removed pressure regulator to make sure it’s not broken (I’ve seen them break, causing oil to bypass). If it’s fine, then the pump itself is needing replacement or a rebuild.
Most people just replace the entire pump without checking anything and go back to work as downtime and failures cost more money than the pump is worth.
I check the things above as sometimes it’s just a simple thing and the pump is working again.
But if dump trucks are busy all the time and needed back to work ASAP, the pump is on the ground and a new one is in it’s place.