The filter on my truck is on the return line. On an end dump, the hydraulic oil flows through the suction line to the pump to the cylinder to raise the bucket, and then reverses flow back through the pump & suction line to the tank....your basic 2-line wet kit. Side dumps use hydraulic pressure both to extend AND retract the hydraulic cylinder. The oil always flows from the tank, through the suction line, to the pump, where it is routed to a reversing valve (which can be mounted on the truck, but is usually located on the trailer), and back to the tank through a return line. Inside the cab, you'll have an on-off-on switch to control the reversing valve...in the "off" position, the hydraulic oil is cycled back to the tank. Push the switch one direction and the valve sends the oil to extend the cylinders. Push the switch the other way and the valve sends the oil to retract the cylinders. So, a filter on the return line isn't going to have any backwash, because the oil is only returning to the tank. Nothing, except the oil would not be filtered...any contaminants which may be in the oil will remain in the oil to be recirculated. It is not a necessary component, and I'm probably the only truck in the fleet that has one....mostly because I wanted it on there because stuff is expensive and I want to prolong the useful life of my stuff because I plan on keeping it for a while.