Ford 5R110W Transmission External Inline Filter 2003-07 136013A The cooler creates a backpressure in the out going line and its pressure is higher than the return line. This variation of fluid pressure is what makes this system work. The in-line filter's media is fine and has a capability of trapping small particles of 2 microns or larger. Even if the filter would become completely clogged, it will not affect the cooler flow because it is parallel to the system. This parallel circuit system provides extreme filtration without the danger of shutting down the cooler flow in the event of the filter becoming clogged. Considering that the in-line filter is parallel to the cooler, it is the only thing that prevents the ATF from bypassing the cooler and simply returning to the transmission assembly. Strictly speaking, if you were to remove the filter from the housing, there would be nothing to force the ATF through the cooler. Granted, the filter is so restrictive, that this is what makes the system work. When flushing the cooler, disconnect the lines at the in-line filter and flush it from there to the cooler, or leave the filter in the housing, flush the system and then replace the filter element. If you flush the cooler (from the transmission lines) with the filter out of its housing, you will bypass the cooler during the flushing process.