Dump slow to lower

proportioning valve. its in part what controls the decent (flow back to tank)

 

Its in the valve. Your pump is fine.

 

Uploaded diagram of pump flow. Your picture appears to be a 2 line installation. If your butterfly connectors are fully opening, next thing to check is spool is going fully to rear. You might need to pull air shift cover off to verify. If oil comes out of air shift cylinder, spool valve end o-ring is leaking and not allowing full travel. If spool valve is shifting fully, next path of fluid is through the combo relief valve. You can pull it and inspect, but it has four 7/16" orfices on the return side that I doubt are plugged. The suppy side has a screen that could plug, but wouldn't cause issue with lowering. Next spot would be your shut-off valve (ensure fully open) then to tank. One of the easiest checks that may be causing your problem is a plugged tank vent. Vent may be separate vent on tank or part of cap. Make sure vent is clean and air easily passes through it. update what you find.

 

It is an integrated relief/load hold valve. Possible, but I doubt that is your problem. I would make sure tank vent is good and verify spool is making full travel 1st.

 

Does the system need to be drained to remove and inspect? Hello I have a similar issue on my tri axle.