The Stovebolt Site Forums The Shop Area Driveline Replacing 4-speed Trans fluid

I personally think you can flush it in neutral, with the engine running. The clutch gear will be turning-as well as the front mainshaft bearing and clutch housing needle bearings, which turns the counter shaft assembly which in turn turns the front and rear bearings on counter shaft, the constant mesh gears on the main shaft, so second and third gears will be rotating on the main shaft. The only thing not moving is the reverse idler and the first/reverse gear and main shaft as well as the rear main shaft bearing, but they will get washed from the whirlpool going on in there. On 1/2 ton trucks I would not recommend this method on enclosed driveshaft because with no seal in the back other than the driveshaft you will wash fluid down into the rear bell u-joint area.