I am pretty sure (I think his name was Dan) that built a 3000 series added case drains. So are looking at the same thing with the trw motors?Most motors are better with case drains, eg. hydrovac. The Ross drive motor used in 200, 400 and 600 series is different. It is a 2-zone motor, there is no way to install case drain. Char-Lynn is a 3-zone motor. Ross was bought by Parker. Here is some Parker info:
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The 2-zone motor has to have a high-pressure shaft seal. The 3-zone motor has to have check valves to vent shaft seal to the low-pressure side. On the 3-zone, if there is too much back pressure or when connected in series, the "case" drain (circle at bottom, unfortunately not labeled) must be connected to tank. Some newer and off-shore 3-zone motors also have high-pressure shaft seals as standard, so case drains are not needed.
The drive isn't last in hydraulic circuit. The auxiliary valve is downstream of motor. On most 600 series, the whole loader (and backhoe) function is downstream.