Doesn't all the hydraulic fluid pass through the pump sooner or later?
Yes of course it does, however, within the pump, a small portion of the fluid is diverted to circulate around the pump for cooling and lubrication. If you happen to be standing around any Boeing and the engine cowlings are open, look at the hydraulic pump. You will see 3 lines. One is supply, one is pressure and the last one is marked case drain. This is the cooling and lubrication drain. That fluid is routed through the fuel tank heat exchanger before returning to the reservoir. The hydraulic fluid used to do the actual work returns directly to the reservoir and is cooled as it passes through the return lines.