I have a Doosan it is fitted with a triplex mast.It lifts ok but when lowering with a load on it goes very slow it does this on all stages take the load off and it is fine.When it goes slow if you raise it then lower fast it is ok. The mast rollers and chains are fine.Have removed the check valve from the cylinder but still the same.Have changed the elec sol valves on the valve block with another truck. The spool in the block is free ,I am thicking it may be a fault in the valve block please help.