I am far from the expert here, but on my ck20s it is an open system so that when the directional control lever is in neutral, the hydraulic fluids are being returned to the reservoir. Normally, A is the input and B is the outlet, although you can reverse the flow with the directional switch. I have yet to figure out why one would want to reverse the flow, but as I said I am no expert in this field. This continuous displacement pump is the reason you want to avoid like the plague, putting the rear hydraulics in gear without there being an implement or bypass hose in place. The pump will overheat and be destroyed.